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

my %seen;           

$/=undef;	    #input record separator doesn't matter any more
my $data = <DATA>;  #whole file is read into one variable!

my @match = ( $data =~ /(first_string|second_string|third_string|fourth_string|fifth_string)/g );
foreach (@match)
{
  $seen{$_}++;  #yes, you can increment a hash key that
                #doesn't exit yet!
} 
#now we have a hash of strings that were seen.
#also know the number times each string was seen.
#but that doesn't appear to be necessary to know that here?

print "".keys(%seen)," matches found: ", join(", ",sort keys(%seen)),"\n";

__DATA__
This is a test file matchme ljldjlfjd l;djfldjlf d
test test test dljfldjlfjldjfldjlljdf one second_string
dlfjldfj  ljdfldjjf ldjfljdl dfljdlfj dfdlfj
three ljfldjlj dlfjlasdj foiidufoiida matchdf
dljfldsaofuoidfousdaof ladsjflasdof first_string
dlfjodsuofuasdo sadoufosadu foasduf aosduf third_string

__END__

__above prints:
3 matches found: first_string, second_string, third_string