I don't have any systems anymore with such old Perl versions, so i'm limited to my rusty old memory and what i can glean from documentation.

I've read through the relevant perldeltas for 5.20 and 5.26. Both had changes regarding handling of literal control characters in variable names.

5.20 added deprecation warnings and 5.26 made code changes to disallow their use.

After seeing this, my best guess (without knowing the perl internals or grepping through the changelogs) is that the small change for 5.20 has introduced some false positives in control characters detection and 5.26 changed the parser enough that those went away.

My second guess would be a change in the Unicode handler, probably in the way Perl or a script detects Unicode/fileending stuff. v5.20 was the upgrade to Unicode 6.3, v5.26 to 9.0.

I could be totally mistaken, though.