Reverse complement of DNA

#!/usr/bin/perl
print"\n\n....Reverse compliment Program....\n\n";
$dna=ATGCTTGGACATAGA;
$rev=reverse $dna;
$rev=~tr/ATGC/TACG/;   # translates all characters of Search List into the corresponding characters of Replacement List.
print "Original dna sequence: $dna \n";
print "Reverse compliment: $rev \n\n";




#For user input

print "\n\t********** GET REVERSE COMPLEMENT OF DNA **********\n\n";
print "PLEASE TYPE THE FILENAME OF THE DNA SEQUENCE := ";
$dnafilename = <STDIN>;
chomp $dnafilename;
unless ( open(DNAFILE, $dnafilename) ) 
{
    print "Cannot open file \"$dnafilename\"\n\n";
    exit;
}
@DNA = <DNAFILE>;
close DNAFILE;
$DNA = join( '', @DNA);
$DNA =~ s/\s//g;
print "\nTHE ORIGINAL DNA SEQUENCE :=\n$DNA\n\n";
@DNA = split( '', $DNA );
print"REVERSE COMPLEMENT OF THE DNA SEQUENCE :=\n";

foreach $nucleotide(reverse(@DNA)) {



    if      ($nucleotide =~ /a/i) {
        print "T";
        print WRITE "T";
    } elsif ($nucleotide =~ /t/i) {
        print "A";
        print WRITE "A";
    } elsif ($nucleotide =~ /g/i) {
        print "C";
        print WRITE "C";
    } elsif ($nucleotide =~ /c/i) {
        print "G";
        print WRITE "G";
    } else {
        die "$0:  Bad nucleotide!  [$nucleotide]\n\n";
    }
}