#!/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;