If initial array has even length, I want to double the length, otherwise I want doubled and -1 length.
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Where $n - is a length of final array ...
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push @array, reverse @array[ 0 .. int( $n / 2 ) - 1 ];

Why bother figuring out the length  $n of the final, palindromatized array when this length is inherent in the initial array?
    push @array, reverse @array[ 0 .. $#array - (@array & 1) ];
(This is essentially vr's solution. (Update: To be quite fair, 1nickt here and poj here also had essentially the same solution, and with earlier precedence.))