Same as the difference between 'single' and "double": one interprets and one doesn't. Though <<'there docs' are unique in Perl for never treating \ as anything other than just \.

If you thought "there doc" was some official term, then perhaps you didn't notice that most of the names used aren't names you'll find often used (never in the standard docs, for example).

Both examples are really "here" docs. The poll author was just being silly.