#! perl
use strict;
use warnings;
use Algorithm::Diff qw( LCS );
use constant {
                 CASE_SENSITIVE  => 0,
                 DICTIONARY_FILE => 'words.txt',
                 MINIMUM_MATCH   => 2,
             };

print "Enter the target word: ";
chomp(my $target = <STDIN>);
my @target = split //, CASE_SENSITIVE ? $target : lc $target;

open(my $in, '<', DICTIONARY_FILE)
    or die "Cannot open file '" . DICTIONARY_FILE . "' for reading: $!";
my %substrings;

while (my $word = <$in>)
{
    chomp $word;
    my @word = split //, CASE_SENSITIVE ? $word : lc $word;
    my @lcs  = LCS(\@target, \@word);
    $substrings{ join('', @lcs) } = $word if @lcs >= MINIMUM_MATCH;
}

close $in
    or die "Cannot close file '" . DICTIONARY_FILE . "': $!";

if (%substrings)
{
    my @matches = sort { length $a <=> length $b } keys %substrings;
    print "The closest match is:  ", $substrings{ $matches[-1] }, "\n";
}
else
{
    print "No matches found\n";
}