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

[jidanni]

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;
}