I'd avoid the sillier approach, it may fail miserably for badly formatted input strings:

#!/usr/bin/perl
use strict;
use warnings;

my $str = '2004';
$str =~ /(.{4})/g;  my $y = $1;
$str =~ /(.{2})/g;  my $m = $1;
$str =~ /(.{2})/g;  my $d = $1;
print "$y-$m-$d\n";

__END__

2004-2004-20
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