I think it's a bug. It has nothing to do with the flip-flop operator and it seems to be caused by jumping out of a block. Consider this example that emulates a flip flop and uses goto instead of next.

#!/usr/bin/perl

use v5.24;
use warnings;

my ($first, $last);
while (<DATA>) {
    chomp;
    $first ||= /d/;
    undef($first) if $last ||= /h/;
    if ($first || $last) {
        undef $last;
        #goto ewhile unless //;
        goto eif unless //;
        say;
        eif:
    }
    ewhile:
}
__DATA__
c
d
e
f
g
h
i
[download]

goto eif:
d
h
[download]

goto ewhile:
d
f
g
h
[download]

Jumping to the end of the current block produces the expected result, while jumping to the end of the while loop reproduces choroba's strange results. The jump out of the block seems to clear the "last successful match" causing // to be taken as an always matching empty pattern.

However, I'd prefer to check the flip-flop's return value as this works in all circumstances, even for if(foo($_) .. bar($_)) {...}.

Update: 26.10.2023

Here is a much simpler example demonstrating the behaviour without any flip-flop behaviour. A jump out of a block transforms the empty pattern // from the last successful matching pattern to a true empty pattern.

#!/usr/bin/perl

use v5.24;
use warnings;

for my $label ('inner', 'outer') {
    say "goto $label";
    for ('c' .. 'g') {
        say "loop: $_";
        say "/d/ matched" if /d/;
        {
            goto $label unless //;
            say "// matched";
            inner:
        }
        outer:
    }
    say '';
}
__DATA__
goto inner
loop: c
// matched
loop: d
/d/ matched
// matched
loop: e
loop: f
loop: g

goto outer
loop: c
// matched
loop: d
/d/ matched
// matched
loop: e
loop: f
// matched
loop: g
// matched
[download]

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