http://www.perlmonks.org?node_id=1059245


in reply to One of these things is not like the others...

From the documentation for push. 
Starting with Perl 5.14, "push" can take a scalar EXPR, which
must hold a reference to an unblessed array. The argument will
be dereferenced automatically. This aspect of "push" is
considered highly experimental. The exact behaviour may change
in a future version of Perl.

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This is probably the answer and note that the docs say it is experimental and may change in the future.

In Section Seekers of Perl Wisdom