You are missing a closing (right) curly for the while loop, as the asymmetric indentation suggests. Here's the perltidy version (Tip #10 from the Basic debugging checklist): 
#!/usr/bin/perl -w
use strict;
use diagnostics;
my $file = "$ARGV[0]";
open( DAT, $file ) || die("Can not open file!");
while (<DAT>) {
    my $seq = $_;
    if ( $seq =~ m/^[ATCG]+$/ ) {
        my $DNA = '';
        $DNA = $seq;
        my $revcom = reverse($DNA);
        $revcom =~ tr/ACGTacgt/TGCAtgca/;
        print $revcom;
    }
}

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I also got rid of the trailing semicolon after your right curly.

In reply to Re: Isolating DNA cont. by toolic
in thread Isolating DNA cont. by undergradguy

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