"... if there are side effects, I did it wrong"

Yes, yes. But you can do what you want: You get the side effect for free - even if you don't want it.

#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dump;

my @original_data_set = ( 1 .. 10 );
dd \@original_data_set;

my @modified_data_set = map { ++$_ } @original_data_set;
dd \@original_data_set, \@modified_data_set;

__END__
karls-mac-mini:monks karl$ ./1119774.pl
[1 .. 10]
([2 .. 11], [2 .. 11])
[download]

Best regards, Karl