How about this. I'd rather look for some module to do it, though...

my @times = qw(7:45:34 19:45:34 23:34:14);

foreach my $t (@times) {
    if ($t =~ /^(\d?\d):(\d?\d):\d?\d$/) {
        my $ampm = "AM";
        my ($h, $m) = ($1, $2);

        if ($h > 12) {
            $h -= 12;
            $ampm = "PM";
        }
        print "$h:$m $ampm\n";
    }
}
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