With some experimenting, I get strange results if the following conditions are all true:

• There is a hash assignment in list context.
• There's a duplicate key on the RHS of the assignment.

Minimal test cases:

$ perl -le '[%h = (1 .. 4)]'   # Fine.
$ perl -le '[%h = (1) x 4]'
Bizarre copy of ARRAY in anonlist at -e line 1.
[download]

Also:

$ perl -le 'print join ".", %h = 1 .. $_ for 1 .. 20'
1
1.2
1.2.3
1.2.3.4
1.2.3.4.5
1.2.3.4.5.6
1.2.3.4.5.6.7
1.2.3.4.5.6.7.8
1.2.3.4.5.6.7.8.9
1.2.3.4.5.6.7.8.9.10
1.2.3.4.5.6.7.8.9.10.11
1.2.3.4.5.6.7.8.9.10.11.12
1.2.3.4.5.6.7.8.9.10.11.12.13
1.2.3.4.5.6.7.8.9.10.11.12.13.14
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19
1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20
[download]

It's interesting to note that the resulting numbers are still in order - what clearly isn't happening is that the list to join is gotten out of the hash (in keys() order). So I guess that some aliasing into the list on the RHS plays a role, some values are probably munged when inserting them into the hash, causing a collision.

Interesting might also be:

$ perl -le 'print join ".", %h = map {(1, $_)} 1 .. $_ for 1 .. 10'
1.1
1..1.2
1.3.1..1.3
1..1.4.1..1.4
1.5.1..1.5.1..1.5
1..1.6.1..1.6.1..1.6
1.7.1..1.7.1..1.7.1..1.7
1..1.8.1..1.8.1..1.8.1..1.8
1.9.1..1.9.1..1.9.1..1.9.1..1.9
1..1.10.1..1.10.1..1.10.1..1.10.1..1.10
[download]

Abigail

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