#!/usr/bin/perl

# http://perlmonks.org/?node_id=1140982

use Algorithm::Diff qw(traverse_sequences);
#use strict;
#use warnings;

open (FH , "<123") or die $!;
open (FH2 , "<456") or die $!;

my @file1 = <FH>;
my @file2 = <FH2>;

sub fullpath
  {
  my @full;
  my @answer;
  for my $line ( shift() =~ /.+\n/g )
    {
    $line =~ /^( *)/;
    $#full = length($1) - 1; # truncate array
    push @full, $line;
    push @answer, join '', grep defined, @full;
    }
  return \@answer;
  }

print @file1;
print @file2;

my $len1 = scalar(@file1);
print "len1 : $len1\n";

my $len2 = scalar(@file2);
print "len2 : $len2\n";

for (my $i==0 ; $i < $len1 ; $i++)
{
  for  (my $j==0 ; $j < $len1 ; $j++)
  {
    if ($i==$j)
    {
my $line1 = $file1[i];
my $line2 = $file2[j];
my $full1 = fullpath($line1);
my $full2 = fullpath($line2);
  }
}
}

traverse_sequences( $full1, $full2,
  {
 # DISCARD_A => sub {print $full1->[shift()], "\n"},
  } );
[download]

    {
my $line1 = $file1[i];
my $line2 = $file2[j];
my $full1 = fullpath($line1);
my $full2 = fullpath($line2);
  }
[download]

"my"s are local to the enclosing block - these six lines do nothing.

Please *never* comment out "use strict" and "use warnings"