perltrap - Perl traps for the unwary


The biggest trap of all is forgetting to use warnings or use the -w switch; see warnings and "-w" in perlrun. The second biggest trap is not making your entire program runnable under use strict. The third biggest trap is not reading the list of changes in this version of Perl; see perldelta.

Awk Traps

Accustomed awk users should take special note of the following:

  • A Perl program executes only once, not once for each input line. You can do an implicit loop with -n or -p.

  • The English module, loaded via

        use English;

    allows you to refer to special variables (like $/) with names (like $RS), as though they were in awk; see perlvar for details.

  • Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.

  • Curly brackets are required on ifs and whiles.

  • Variables begin with "$", "@" or "%" in Perl.

  • Arrays index from 0. Likewise string positions in substr() and index().

  • You have to decide whether your array has numeric or string indices.

  • Hash values do not spring into existence upon mere reference.

  • You have to decide whether you want to use string or numeric comparisons.

  • Reading an input line does not split it for you. You get to split it to an array yourself. And the split() operator has different arguments than awk's.

  • The current input line is normally in $_, not $0. It generally does not have the newline stripped. ($0 is the name of the program executed.) See perlvar.

  • $<digit> does not refer to fields--it refers to substrings matched by the last match pattern.

  • The print() statement does not add field and record separators unless you set $, and $\. You can set $OFS and $ORS if you're using the English module.

  • You must open your files before you print to them.

  • The range operator is "..", not comma. The comma operator works as in C.

  • The match operator is "=~", not "~". ("~" is the one's complement operator, as in C.)

  • The exponentiation operator is "**", not "^". "^" is the XOR operator, as in C. (You know, one could get the feeling that awk is basically incompatible with C.)

  • The concatenation operator is ".", not the null string. (Using the null string would render /pat/ /pat/ unparsable, because the third slash would be interpreted as a division operator--the tokenizer is in fact slightly context sensitive for operators like "/", "?", and ">". And in fact, "." itself can be the beginning of a number.)

  • The next, exit, and continue keywords work differently.

  • The following variables work differently:

          Awk       Perl
          ARGC      scalar @ARGV (compare with $#ARGV)
          ARGV[0]   $0
          FILENAME  $ARGV
          FNR       $. - something
          FS        (whatever you like)
          NF        $#Fld, or some such
          NR        $.
          OFMT      $#
          OFS       $,
          ORS       $\
          RLENGTH   length($&)
          RS        $/
          RSTART    length($`)
          SUBSEP    $;
  • You cannot set $RS to a pattern, only a string.

  • When in doubt, run the awk construct through a2p and see what it gives you.

C/C++ Traps

Cerebral C and C++ programmers should take note of the following:

  • Curly brackets are required on if's and while's.

  • You must use elsif rather than else if.

  • The break and continue keywords from C become in Perl last and next, respectively. Unlike in C, these do not work within a do { } while construct. See "Loop Control" in perlsyn.

  • The switch statement is called given/when and only available in perl 5.10 or newer. See "Switch Statements" in perlsyn.

  • Variables begin with "$", "@" or "%" in Perl.

  • Comments begin with "#", not "/*" or "//". Perl may interpret C/C++ comments as division operators, unterminated regular expressions or the defined-or operator.

  • You can't take the address of anything, although a similar operator in Perl is the backslash, which creates a reference.

  • ARGV must be capitalized. $ARGV[0] is C's argv[1], and argv[0] ends up in $0.

  • System calls such as link(), unlink(), rename(), etc. return nonzero for success, not 0. (system(), however, returns zero for success.)

  • Signal handlers deal with signal names, not numbers. Use kill -l to find their names on your system.

JavaScript Traps

Judicious JavaScript programmers should take note of the following:

  • In Perl, binary + is always addition. $string1 + $string2 converts both strings to numbers and then adds them. To concatenate two strings, use the . operator.

  • The + unary operator doesn't do anything in Perl. It exists to avoid syntactic ambiguities.

  • Unlike, Perl's for (also spelled foreach) does not allow the left-hand side to be an arbitrary expression. It must be a variable:

       for my $variable (keys %hash) {

    Furthermore, don't forget the keys in there, as foreach my $kv (%hash) {} iterates over the keys and values, and is generally not useful ($kv would be a key, then a value, and so on).

  • To iterate over the indices of an array, use foreach my $i (0 .. $#array) {}. foreach my $v (@array) {} iterates over the values.

  • Perl requires braces following if, while, foreach, etc.

  • In Perl, else if is spelled elsif.

  • ? : has higher precedence than assignment. In JavaScript, one can write:

        condition ? do_something() : variable = 3

    and the variable is only assigned if the condition is false. In Perl, you need parentheses:

        $condition ? do_something() : ($variable = 3);

    Or just use if.

  • Perl requires semicolons to separate statements.

  • Variables declared with my only affect code after the declaration. You cannot write $x = 1; my $x; and expect the first assignment to affect the same variable. It will instead assign to an $x declared previously in an outer scope, or to a global variable.

    Note also that the variable is not visible until the following statement. This means that in my $x = 1 + $x the second $x refers to one declared previously.

  • my variables are scoped to the current block, not to the current function. If you write {my $x;} $x;, the second $x does not refer to the one declared inside the block.

  • An object's members cannot be made accessible as variables. The closest Perl equivalent to with(object) { method() } is for, which can alias $_ to the object:

        for ($object) {
  • The object or class on which a method is called is passed as one of the method's arguments, not as a separate this value.

Sed Traps

Seasoned sed programmers should take note of the following:

  • A Perl program executes only once, not once for each input line. You can do an implicit loop with -n or -p.

  • Backreferences in substitutions use "$" rather than "\".

  • The pattern matching metacharacters "(", ")", and "|" do not have backslashes in front.

  • The range operator is ..., rather than comma.

Shell Traps

Sharp shell programmers should take note of the following:

  • The backtick operator does variable interpolation without regard to the presence of single quotes in the command.

  • The backtick operator does no translation of the return value, unlike csh.

  • Shells (especially csh) do several levels of substitution on each command line. Perl does substitution in only certain constructs such as double quotes, backticks, angle brackets, and search patterns.

  • Shells interpret scripts a little bit at a time. Perl compiles the entire program before executing it (except for BEGIN blocks, which execute at compile time).

  • The arguments are available via @ARGV, not $1, $2, etc.

  • The environment is not automatically made available as separate scalar variables.

  • The shell's test uses "=", "!=", "<" etc for string comparisons and "-eq", "-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which uses eq, ne, lt for string comparisons, and ==, != < etc for numeric comparisons.

Perl Traps

Practicing Perl Programmers should take note of the following:

  • Remember that many operations behave differently in a list context than they do in a scalar one. See perldata for details.

  • Avoid barewords if you can, especially all lowercase ones. You can't tell by just looking at it whether a bareword is a function or a string. By using quotes on strings and parentheses on function calls, you won't ever get them confused.

  • You cannot discern from mere inspection which builtins are unary operators (like chop() and chdir()) and which are list operators (like print() and unlink()). (Unless prototyped, user-defined subroutines can only be list operators, never unary ones.) See perlop and perlsub.

  • People have a hard time remembering that some functions default to $_, or @ARGV, or whatever, but that others which you might expect to do not.

  • The <FH> construct is not the name of the filehandle, it is a readline operation on that handle. The data read is assigned to $_ only if the file read is the sole condition in a while loop:

        while (<FH>)      { }
        while (defined($_ = <FH>)) { }..
        <FH>;  # data discarded!
  • Remember not to use = when you need =~; these two constructs are quite different:

        $x =  /foo/;
        $x =~ /foo/;
  • The do {} construct isn't a real loop that you can use loop control on.

  • Use my() for local variables whenever you can get away with it (but see perlform for where you can't). Using local() actually gives a local value to a global variable, which leaves you open to unforeseen side-effects of dynamic scoping.

  • If you localize an exported variable in a module, its exported value will not change. The local name becomes an alias to a new value but the external name is still an alias for the original.

As always, if any of these are ever officially declared as bugs, they'll be fixed and removed.