Although not the most efficient, this seems to do the job:

while ($parse =~ s!(\d{1,2})/(\d{1,2})(?:-(\d{1,2}))?!!) {
  $end = $3 ? $3 : $2;
  print "untagged Interface GigabitEthernet $1/$_\n" for ($2 .. $end)
}
[download]

Test program:

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

while (my $line = <DATA>) {
  chomp($line);
  while ($line =~ s!(\d{1,2})/(\d{1,2})(?:-(\d{1,2}))?!!) {
    my $slot = $1;
    my $port_start = $2;
    my $port_end = $3 ? $3 : $port_end;
    for my $port ($port_start .. $port_end) {
        print "untagged Interface GigabitEthernet $slot/$port\n";
    }
  }
}

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