It would be helpful if you could give say 5-6 examples of what you want, meaning input strings and desired output.

The general problem of how to compare text and determine "how close" one string is to another is very complex.

I'm not quite sure, but I think you just need some code that points out where the 2 strings diverge (first char that doesn't match). The simple code below is a step in that direction. I'm sure you will need to modify this to your needs, but I hope this gets you started on the "path".

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

my $string1 = "abcdf";
my $string2 = "abcde";

#for this simple comparision, I assume that using
#the longest string as the iterator value
#which will be "string1", is "better".

if ( length($string2) > length ($string1) )
{
   ($string1,$string2) = ($string2,$string1);
   #in Perl an intermediate "temp" is not necessary
}

my @string2 = split (//,$string2); #array of characters
my $result="";
foreach (split (//,$string1)) #loop var is $_ over chars in
                              #string1
{
   my $char = shift (@string2);
   if (defined ($char) && $char eq $_ )
   {
      $result .= "1";
   }
   else
   {
      $result .= $_;
   }
}
print "CharbyCharResult =$result\n";
print "difference is:",grep{!/1/}split(//,$result),"\n";

__END__
EXAMPLE RUNS:

my $string1 =    "abcdf";
my $string2 =    "abcde";
CharbyCharResult =1111f
difference is:f

my $string1 =    "abcdfccc";
my $string2 =    "abcde";
CharbyCharResult =1111fccc
difference is:fccc

my $string1 =    "abcdf";
my $string2 =    "abcdeeeee";
CharbyCharResult =1111eeeee
difference is:eeeee

my $string1 =    "abcdfxy";
my $string2 =    "abcdexzy";
CharbyCharResult =1111e1zy
difference is:ezy
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