Operands are evaluated left-to-right except for assignment operators. Even right-associative ** evaluates its operands left-to-right. It's not documented, but I don't see it changing. From what I've read, I'd even say there's a desire to fix a case where it isn't, should one be found.

If you wanted to avoid relying on the undocumented feature, you'd do

sub push (\@;@) {
    my $array = shift;
    CORE::push ( @$array, @_ ? @_ : $_ )
}
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Of course, both yours and this modified version fail to handle @b=(); push @a, @b; properly.