File::Find will handle recursive directory spanning.

Here's a simple sub which takes a filepath and a string, returning true if the file contains the string. Slurp is used, so files should be smallish.

sub contains {
    my ($file, $string) = @_;
    my $content;
    {
        local $/;
        open my $fh, '<', $file or die $!;
        $content = <$fh>;
        close $fh or die $!;
    }
    index( $content, $string) == -1
      ? 0
      : 1;
}
[download]

That should be fairly quick.

Update: Any sub returns the value of its last expression unless an explicit return comes before. In sub contains, the trinary operator ?: tests whether index returned -1, and evaluates to zero if so, otherwise evaluates to 1. That could as well have been written     return not index( $content, $string) == -1 or     return index( $content, $string) != -1 where the return keyword is optional.

After Compline,
Zaxo