You might be better off using a module; that way, you don't have to deal with all the edge cases.

#!/usr/bin/perl -w
use strict;
use Time::Simple;

for (<DATA>){
    s/(..)$/:$1/;
    my $former_time = Time::Simple->new($_);
    my $new_time = $former_time + 15 * 60;
    print "former_time is $former_time\nnew time is $new_time\n";
}

__DATA__
1250
0549
1103
1509
0423
0834
[download]

Output:

former_time is 12:50:00
new time is 13:05:00
former_time is 05:49:00
new time is 06:04:00
former_time is 11:03:00
new time is 11:18:00
former_time is 15:09:00
new time is 15:24:00
former_time is 04:23:00
new time is 04:38:00
former_time is 08:34:00
new time is 08:49:00
[download]