my $new_retid = $retid;
  $new_retid .= '_001' unless $retid =~ /_\d{3}$/;

##</code><code>##

  my $new_retid = $retid . '_001' x $retid !~ /_\d{3}$/;

##</code><code>##

  (my $new_retid = $retid) .= do {'_001' unless $retid =~ /_\d{3}$/};

##</code><code>##

  my $new_retid = $retid . ($retid !~ /_\d{3}$/ and '_001');

##</code><code>##

(my $new_retid = $retid) =~ s/(?<!_\d{3})$/_001/;

##</code><code>##

my ($new_retid) = grep( $_, $retid =~ /(^\d{4}_\d{3})$/, $retid.'_001');