Below is a simple solution with little perl code and relying on standard UNIX tools (find and grep). grep -F is used for fast (fixed string instead of regexp) search. It searches all the files in $DIR and all its subdirectories and obtains the names of those matching $STRING. Note that Perl 5.8.0+ is required (for the safe version of open). If you don't have it you must do the shell escaping yourself.

#!/usr/bin/perl

# safe form of IPC open requires perl 5.8.0
use v5.8.0;

my $DIR = '/some/directory';
my $STRING = 'hidden!';

open my $fh, '-|', 'find', $DIR, qw/-type f -exec grep -lF/, $STRING, 
+qw/{} ;/
  or die $!;

chomp (my @found = <$fh>);

# @found now contains the list of files matching the string;
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