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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For extra credit, leave out the "1 ==" part ;-)