Not the most elegant solution, and probably a lot more verbose than it needs to be - but appears to give the desired result...

#!/usr/bin/perl
use warnings;
use strict;

while (my $line = <DATA>) {
    chomp($line);
    my @parts = split /\s/, $line, 4;
    for (split /,/, $parts[3]) {
        my ($slot, @ports) = split /\//;
        if ($ports[0] =~ /(\d+)-(\d+)/) {
            @ports = ($1 .. $2);
        }
        for my $port (0 .. $#ports) {
            print join(' ', @parts[0..2], "$slot/$ports[$port]\n");
        }
    }
}

__DATA__
untagged Interface GigabitEthernet 1/2,1/3-5,1/12,1/40
[download]

Output:

untagged Interface GigabitEthernet 1/2
untagged Interface GigabitEthernet 1/3
untagged Interface GigabitEthernet 1/4
untagged Interface GigabitEthernet 1/5
untagged Interface GigabitEthernet 1/12
untagged Interface GigabitEthernet 1/40
[download]

I'd suggest testing with more data before using it.
Hope this helps,
Darren :)