Can't find string terminator "'" anywhere before EOF

The test suite fails with perl 5.41.x (e.g. 5.41.4):
```
Getopt::Euclid: Invalid .default value: step.default: $STEP1 + $STEPS->{extra} + $$STEPS{extra} + ${$STEPS}{extra} + $main'STEP2
(Can't find string terminator "'" anywhere before EOF at (eval 11) line 1.
) at t/defer.t line 61.
# Looks like your test exited with 255 just after 2.
t/defer.t .......................
Dubious, test returned 255 (wstat 65280, 0xff00)
All 2 subtests passed
```
This looks like the removed old-fashioned package separator. Probably replacing it with `::`` would fix the problem (not tested).

Start by running t/defer.t with Perl 5.41.x and inspect the failure at line 61, focusing on the reported default value and its package-separator syntax. Trace where that value is evaluated and verify that the test suite passes without the string-terminator error after the compatibility issue is addressed.

Written by the indexing model from the issue text.