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
[download]

This could also be written without regex, since you only really care about one character difference. Just match the characters both ways if there is one character difference in length, and if the matched characters total is equal to the length of the smaller string, then there's only one character difference.

adbc abc
a eq a -> 1
d ne b ->
c eq c <- 2
b eq b <- 3
d ne a <-
[download]

Total matches = length of abc.