Like this:

try.pl:

#!/usr/bin/perl -w
use strict;

my $count = 0;

while (<>) {
    $count += s/foo/bar/g;
    print;
}

print "Nothing found!\n" unless $count;
[download]

Show it in action:

H:\>cat try1.txt
bla
blurb
foo foo
foo
lilalu

H:\>cat try2.txt
bla
blurb
fuu fuu
fuu
lilalu

H:\>cat try1.txt | perl try.pl
bla
blurb
bar bar
bar
lilalu

H:\>cat try2.txt | perl try.pl
bla
blurb
fuu fuu
fuu
lilalu
Nothing found!

H:\>
[download]

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com