Here's a variant on the above, but syntactically clearer:

#!/usr/bin/env perl

use strict ;
use warnings ;

## DNA string
my $string = "aaaggctt";

## syntactically clear hash
my %hash = ( 
    "a" => "t" , 
    "g" => "c" , 
    "c" => "g" , 
    "t" => "a" 
);

## print the DNA string and reverse complement
## add some extra spacing to make it look pretty
print "\n" ; 
print "\t        The DNA string is:  " . $string . "\n" ;
print "\tThe reverse complement is:  " ;


## go through the string one element at a time 
## and print reverse complement 

for my $i (0..length($string)-1) {
    print $hash{ substr($string, $i, 1) } ; 
}

## finish with a little extra spacing
print "\n\n" ;
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