Here you go. I included some error checking in case someone mixed letters in with the number, or left out the numerical part entirely:
use strict;
use warnings;
print "How long should I sleep, in hours? ";
my $wait = <STDIN>; ($wait) = $wait =~ /(\d+)/; $wait -= 0;
my $end = time() + $wait * 3600;
sleep 1 while time() < $end;
print "FINISHED";