Declare Just SP. This is actually identical to dSP, and declares a local copy of perl's stack pointer, available via the SP macro. See "SP" in perlapi. (Available for backward source code compatibility with the old (Perl 5.005) thread model.)
SP
dSP
"SP" in perlapi
To install less, copy and paste the appropriate command in to your terminal.
cpanm
cpanm less
CPAN shell
perl -MCPAN -e shell install less
For more information on module installation, please visit the detailed CPAN module installation guide.