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use strict;
use warnings;
...
my ( $n,  %h ) = -1;
$_ = $h{$_} //= $n+=2 for @a;
print Dumper \@a;

my ( $n,  %h ) = -1;
$_ = $h{$_} //= $n+=2 for @a;

$_=$h{$_}//=$}+=2-!$}for@a;