This is significantly faster than using eval:

sub count
{
        my ($string, $char) = @_;
        my $index = -1;
        my $count = -1;
        do {
                $count++;
                $index = index($string, $char, $index + 1);
        } while ($index != -1);
        return $count;
}
[download]

And this is faster still:

sub count
{
        my ($string, $char) = @_;
        $string =~ s/[^$char]//g;
        return length($string);
}
[download]

        Eval: 56 wallclock secs (45.19 usr +  0.00 sys = 45.19 CPU) @ 
+3319.32/s (n=150000)
  While Loop:  5 wallclock secs ( 4.69 usr +  0.00 sys =  4.69 CPU) @ 
+31982.94/s (n=150000)
Substitution:  1 wallclock secs ( 1.41 usr +  0.00 sys =  1.41 CPU) @ 
+106382.98/s (n=150000)
[download]

Update (7/19/00): Oh, duh. s/// returns a count of the number of changes made. Here's a faster way to write the subroutine:

sub count
{
    my ($string, $char) = @_;
    return ($string =~ s/$char//g);
}
[download]

- Matt