But why not a simple one-liner:

Win8 Strawberry 5.8.9.5 (32)  Thu 07/28/2022  8:22:01
C:\@Work\Perl\monks
>perl  -MData::Dump  -e "my @m = map { ($_) x 2 } @ARGV;  dd \@m"  a b
+ c
["a", "a", "b", "b", "c", "c"]
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