Find element in array

[Sofie]

Hi I am very new to perl and struggling with simple things... I am trying to check if an input DNA sequence only contains nucleotides. And if it doesn't I want to print out the position in the sequence where an invalid character was entered. This is as far as I have come:

#!/usr/bin/perl -w

$DNA = <STDIN>;
chomp ($DNA);

@DNA = split ("", $DNA);
$lengthseq = scalar @DNA;
print "The length of the sequence is:\n", $lengthseq, "\n";

@nucleotideDNA = "";

#check if each element in array is nucleotide
foreach $nucleotide (@DNA){
    if ($nucleotide =~ /^[ATCG]+$/){
        push @nucleotideDNA, $nucleotide;
        }
    else {
        push @nonvalid, $nucleotide;
        }
    }

But how can I print the position of the non valid character? Not sure if this makes any sense.. Thanks