I'd throw in some stricter error checking and write it like this:

$retid =~ /\A(\d{4})(_\d{3})?\z/
    or die "Malformed ret. id\n";

my $new_retid = $1 . ( $2 ? $2 : '_001' );
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Though unless you need it, I'd just change with the original value:

$retid .= '_001' if not $2;
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Update: s/\Q\d{4}/(\\d{4})/ of course. Thanks catching this to Not_a_Number.