Can also use a regex or substitution...

my $string = '20050516';

my $newstring;
$newstring = $2 . $1 if $string =~ /(\d{4})(\d{4})/;
warn $newstring;

(my $newstring = $string) =~ s/(\d{4})(\d{4})/$2$1/;
warn $newstring;
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Could also use a more robust regex if desired (trivial example is /(200[012345])(\d{4})/) and depending on context.