If it's just hours and minutes, you could write a subroutine:

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

my $time = "12:30";
my $secondTime = minutes($time, 120);

print STDERR "TIME: $time\tNEW: $secondTime\n";

sub minutes
    {
        my $extraHour = 0;
        my $time = $_[0];
        my $minutes = $_[1];
        my @time = split(/:/, $time);
        my $modMinutes = $minutes%60;
        $extraHour = 1 if $minutes > $time[1];
        my $hours = $extraHour + (int($minutes/60));
        my $newHour = sprintf("%02d", ($time[0] - $hours)%24);
        my $newMinutes = sprintf("%02d", ($time[1] - $modMinutes)%60);
        my $newTime = "$newHour:$newMinutes";
        return $newTime;
    }
[download]

EDIT: changed extra hour.