It has nothing specifically to do with POSTMATCH, you just ran into the multi-concatenation optimization.

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11118049
use warnings;
use v5.10;

sub add_incr_suffix
{
  state $suffix = 'A';
  return "prefix-TEXT-" . $suffix++;
}

print 'Test ' .
    ( add_incr_suffix =~ /^prefix-*/p ? "${^POSTMATCH}" : '' ) . "," .
    ( add_incr_suffix =~ /^prefix-*/p ? "${^POSTMATCH}" : '' ) . "," .
    ( add_incr_suffix =~ /^prefix-*/p ? "${^POSTMATCH}" : '' ) . "\n";
[download]

Here is a simpler example that produces different output under 5.26 and 5.30

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11118049
use warnings;

my $foo;

print "test: " . ($foo = 1) . ($foo = 2) . ($foo = 3), "\n";
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.