my $seq="ACGGGAGGACGGGAAAATTACTACGGCATTAGC";

my %xref = ( 
  A => 'T', C => 'G', G => 'C', T => 'A',
  a => 't', c => 'g', g => 'c', t => 'a',
);

for(my $ix = 0; $ix < length $seq; ++$ix ) {
    substr( $seq, $ix, 1, $xref{ substr( $seq, $ix, 1 ) } );
}

my $reverse = reverse($seq);
print $reverse, "\n";