bug#5064: 23.1.50; perl mode coloring gone if subroutine name is one letter long

Gentlemen, consider the following perl subroutines.
Notice how the latter has all its coloring gone, just because it starts
with one letter. (perl-mode)^X^E

sub max {
    my $max = shift(@_);
    foreach $foo (@_) {
        $max = $foo if $max < $foo;
    }
    return $max;
}
sub m {
    my $max = shift(@_);
    foreach $foo (@_) {
        $max = $foo if $max < $foo;
    }
    return $max;
}