Text::Levenshtein implements a well-known metric for difference between strings.

$ perl -MText::Levenshtein=distance -e'print distance( "foo bar baz","
+fooo bar baz"),$/'
1
$
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So your off-by-one test could be written,

use Text::Levenshtein qw/distance/;
sub off_by_one {
    1 == distance("foo bar baz", shift);
}
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Text::Levenshtein (which also has an XS version) will give the "edit distance" between two strings. There are several other modules that you could consider, including Text::PhraseDistance (which takes into consideration word order within the string) and String::Approx. Search CPAN - I'm sure there are others.

HTH

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/;
}
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use strict;
use warnings;

my $str = 'foo bar baz';

while (<DATA>) {
    chomp;
    print "$_\n" if ob1($str, $_);
}

sub ob1 {
    my ($w1, $w2) = @_;
    return 0 if (abs(length($w1) - length($w2)) > 1);
    return 1 if $w1 eq $w2;
    if (length($w1) == length($w2)) {
        my ($i, $c);
        for ($i = 0; $i < length($w1); $i++) {
            $c++ if substr($w1, $i, 1) ne substr($w2, $i, 1);
            return 0 if $c && $c > 1;
        } return 1;
    }
    if (length($w1) > length($w2)) {
        my $t = $w1; $w1 = $w2; $w2 = $t;
    }
    $w1 = join '.?', '', split(//, $w1), '';
    return 1 if $w2 =~ /$w1/;
    return 0;    
}

__DATA__
fooo bar baz
foo bar baaz
foo bbar baz
foo bar baz
foo bar bz

fob bar biz
fooo bar bazz
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adbc abc
a eq a -> 1
d ne b ->
c eq c <- 2
b eq b <- 3
d ne a <-
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