This is definetely a problem to go to CPAN for, but in the interest of wheel-reinvention, here is an implementation of my interpretation of your question. The function below matches "sticky-finger" typos, i.e. it matches strings where any number of characters were duplicated (up to a specifiable number of times). That seemed to be the gist of your question... maybe you could provide strings that shouldn't match, too?

use strict;
use warnings;

my $input = 'foo bar baz';
my @test_strings = (
    'fooo bar baz',
    'foo bar baaz',
    'foo bbar baz',
    'ffoo baar baz', # Matches:  Should it?
    'fxo bar baz',   # No match: Different character, not dup.
    'foo baaar baz'  # No match: Too many 'a's!
);
my $fuzziness = 1;

# Let's try out the function
for (@test_strings){
    if (fuzzy_match($input, $_, $fuzziness)){
        print "MATCHED:  $_\n";
    } else{
        print "NO MATCH: $_\n";
    }
}

sub fuzzy_match{
    my ($input, $test_string, $fuzziness) = @_;

    # Build a regex from the input string
    my $regex = '';
    for (split //, $input){
        $regex .= quotemeta($_) . "\{1," . ($fuzziness + 1) . "}?";
    }

    $test_string =~ m/$regex/;
}
[download]

As for the number of characters different, I think that would be length($test_string) - length($input_string).

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