Here is one way to do it, inspired by number sequence.

use strict;
use warnings;

my $in = '1/2,1/3-5,1/12,1/40';

my @munged =
    map
    {
        my ( $slot, $portstr ) = split( '/', $_ );
        my @ports = ( $portstr =~ m/(\d+)-(\d+)/ ) ? $1 .. $2 : $ports
+tr;
        map { join( '/', $slot, $_ ) } @ports;
    } split( ',', $in );

print "$_\n" for @munged;
[download]

It could be shortened, of course, if length is an issue.

You could also play with Set::IntSpan (as I mentioned in the aforementioned thread).