Perl has a special conditional operator (the ternary ?:) which is custom made for just this type of thing.

my $T = ($hour > 12) ? "PM" : "AM";
[download]

update: Whoops, just noticed that the OP is also decreasing $hour for the "PM" case. In that case, the ternary operator becomes a little more ugly.

my $T = ($hour > 12) ? do {$hour-=12; "PM"} : "AM";
[download]

Your best bet is probably one of the CPAN modules like dragonchild recommends.