$stamp = 'Wed 08/22/2001 2:38p';
my ($month,$day,$year,$time) = ($stamp =~ m|(\d+)/(\d+)/(\d+)\s+([\d:]+[pa])|);
$time =~ s/://;

my $newstamp = join('_',$month,$day,$year,$time);

print "$stamp => $newstamp\n";