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Perl::Critic::Policy::InputOutput::ProhibitTwoArgOpen - Write open $fh, q{<}, $filename; instead of open $fh, "<$filename";.


This Policy is part of the core Perl::Critic distribution.


The three-argument form of open (introduced in Perl 5.6) prevents subtle bugs that occur when the filename starts with funny characters like '>' or '<'. The IO::File module provides a nice object-oriented interface to filehandles, which I think is more elegant anyway.

  open( $fh, '>output.txt' );          # not ok
  open( $fh, q{>}, 'output.txt' );     # ok

  use IO::File;
  my $fh = IO::File->new( 'output.txt', q{>} ); # even better!

It's also more explicitly clear to define the input mode of the file, as in the difference between these two:

  open( $fh, 'foo.txt' );       # BAD: Reader must think what default mode is
  open( $fh, '<', 'foo.txt' );  # GOOD: Reader can see open mode

There is also a one-argument form of open which retrieves the expression to open from the global variable with the same name as the handle, but this has the same problems as the two-argument form, and adds in more ambiguity.

  our $FH = '<foo.txt';
  open( FH ); # not ok

This policy will not complain if the file explicitly states that it is compatible with a version of perl prior to 5.6 via an include statement, e.g. by having require 5.005 in it.


This Policy is not configurable except for the standard options.


There is one case in which you are forced to use the two-argument form of open: when doing a safe pipe open, as described in perlipc.





Jeffrey Ryan Thalhammer <>


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