Is there some compelling reason to make this a one-line assignment? Here's what I'd consider nicely readable:

  my $new_retid = $retid;
  $new_retid .= '_001' unless $retid =~ /_\d{3}$/;
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Apart from the ternary operator suggested by several, here are a few bletcherous but possibly instructive ways to turn it into one assignment:

  my $new_retid = $retid . '_001' x $retid !~ /_\d{3}$/;
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  (my $new_retid = $retid) .= do {'_001' unless $retid =~ /_\d{3}$/};
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  my $new_retid = $retid . ($retid !~ /_\d{3}$/ and '_001');
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Update:
One more:

(my $new_retid = $retid) =~ s/(?<!_\d{3})$/_001/;
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And one inspired by delerium's post above:

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