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use strict;
use warnings;
...
my $rev='';
my $n = length $seq; while( $n-- ){ $rev .= $_ for map chr($_&2?$_^4:$
+_^21), ord substr $seq, $n, 1 }
print "$seq\n$rev\n";

use strict;
use warnings;
...
my $rev='';
$rev = "$_$rev" for map chr($_&2?$_^4:$_^21), map ord, split//,$seq;
print "$seq\n$rev\n";