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]