#file: test.pl
#!/usr/bin/perl -w
use strict;
my $pattern='test';
my $output='$&';
while(<DATA>)
{
    chomp;
    my $line=$_;
	chomp($line);
	$line=~/$pattern/;
    #print "$line:".$'."\n";   #why uncommenting this line make the following line pass without no warning.
	my $result="$line:".eval($output)."\n";
	print $result;
}

__DATA__
test1a.txt
test1b.txt
test1c.txt