#!/usr/local/bin/perl -w

use strict;

print "Name of file containing various random strings?\n";
chomp (my $file = <STDIN>);
open (MOTIF, "$file") || die "$!";

print "String to search with?\n";
chomp (my $blocks = <STDIN>);

my $motif;
{
    local $/;
    $motif = <MOTIF>;
}

my $i = 1;
my $re = substr($blocks, 0, $i);

while ($i <= length $blocks and $motif =~ /\Q$re/) {
    $re = substr($blocks, 0, ++$i);
}

chop $re if $i <= length $blocks;

print "$re\n";