To see if you have a number

perldoc -f number -> perlfaq4 -> How do I determine whether a scalar is a number/whole/integer/float? -> http://perldoc.perl.org/Scalar/Util.html#looks_like_number

To quiet not-numeric warning, don't numify empty strings, they're false, so promote them to zero   $num ||= 0;

Instead of numbered variables ($cmd1...) use an array, @commands

Exit code is often used to communicate failure, so you probably don't have to guess based on whats a program prints. my preferred way to qx is with Capture::Tiny

#!/usr/bin/perl --
use strict;
use warnings;
use Capture::Tiny qw/ capture /;
my @cmd = ( 'python', '-V' );
my( $stdout, $stderr, $exit ) = capture {
     system { $cmd[0] } @cmd;
};;
$exit and die "it failed with $exit and $stderr ";
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