Perl Security: 3. Reporting warnings
Reporting warnings with Perl < 5.6.0

With older Perl versions, the report of warnings off by default, and will be turned on if the special variable $^W has a true value

This variable can be set or modified using command line switches when invoking Perl:

-w: Start with $^W = 1;

-W: Always report warnings, regardless of $^W setting

-X: Never report warnings, regardless of $^W setting

You can specify the switches in different ways:

perl -w myprogram.pl

With #!/usr/bin/perl -w at myprogram.pl's beginning