I'm posting this just because the idiom is nifty, not because I want to help with your homework.

push @{$_ & 1 ? \ @odd : \ @even }, $_ for @nums;
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Added

@nums = 1..20;
/.*(?=.)([13579]$)?(?(1)(?{${"\@nums::${_}::"}++})|(?{${"\@nums::${_}:
+:"}--}))/ for @nums;
printf "%d %s\n", $_, ${"\@nums::$_"} > 0 ? odd : even for keys %{"\@n
+ums::"};
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