I kind of like the below, as it gets past the multiple dots in a file name (like test.doc.doc):

#!/usr/bin/perl -w
use strict;
my @files=qw(blah.doc.doc
             blah.doc.txt
             blah.doc);
my ($d, $m, $y) = (localtime)[3 .. 5];
++$m;
$y += 1900;
my $date = "$m$d$y";

foreach (@files){
    s/(.*)(\..*)/$1_$date$2/;
    print "$_\n";
}
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