my $new_retid = $retid . ($retid =~ /_\d{3}\z/ ? '' : '_001');
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Thanks to All!

Please don't assume that someone else is right just because they are more experienced than you; always try (as you have done :) to get an explanation. Cargo cultism often stems from unquestioningly assuming that unusually-written code is that way for a reason.

As to your inquiry, '\z' matches the very end of the string; whereas '$' matches the end, or just before a newline at the end of a string.
This doesn't make any difference in your case; so I'd use '$' to avoid confusion.

If you're curious; '\z' becomes much more useful when you switch on the '/m' (multi-line) flag on a regex to allow '$' to match before any newlines in the string. Have a look at perldoc perlre or Mastering Regular Expressions for more details.

This doesn't make any difference in your case; so I'd use '$' to avoid confusion.

IMO, using $ causes confusion.
• To those who know the difference between \z and $, it confuses what the code does
• To those who don't, it perpetuates their confusion.

  Note that \Z (capital) is equivalent to (?-m:$) (i.e. does what $ does not under //m) even when //m is on. \z (lowercase) always is just the end of the string.

With $, it would match either "9999_999\n" or "9999_999", which is not what your original post requested.