( my $new = reverse $string ) =~ tr/atgc/tacg/;
print "$new\n";

[download]

Update: Also if you want to remove any non a, t, c or g characters: 

( my $new = reverse $string ) =~ tr/atgc\0-\277/tacg/d;
print "$new\n";

[download]

In reply to Re: char substitution without regexes by jwkrahn
in thread char substitution without regexes by biohisham

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