Using

#!/usr/bin/perl

use strict;
use warnings;

use v5.36;

say 'Hello, world!';
[download]

In 5.38, we see:

  DB<1> use Devel::Peek;

  DB<2> Dump( ${"_<a.pl"}[8] );
SV = PVMG(0x55d8627958e0) at 0x55d86251ff80
  REFCNT = 1
  FLAGS = (IOK,POK,IsCOW,pIOK,pPOK)
  IV = 94387850872920
  NV = 0
  PV = 0x55d8628074b0 "say 'Hello, world!';\n"\0
  CUR = 21
  LEN = 32
  COW_REFCNT = 1

  DB<3> Dump( ${"_<a.pl"}[6] );
SV = PVMG(0x55d862786d50) at 0x55d86251fec0
  REFCNT = 1
  FLAGS = (POK,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x55d8627d7d50 "use v5.36;\n"\0
  CUR = 11
  LEN = 16
[download]

See how "line 8" has IOK and POK, but "line 6" only has POK? That means that line 8 contains an integer in addition to the string, while line 6 contains only the string. This doesn't match the documentation.

(As a side note, why 94387850872920? In hex, it's 55D862528858, which in the same range as the addresses being dumped. So it looks like it's not just a flag, but a pointer to something.)

Note that the problem is limited to lines containing pragmas. The non-pragma unbreakable lines have IOK and a value of zero.

  DB<4> use Devel::Peek;Dump( ${"_<a.pl"}[5] );
SV = PVMG(0x55856a4198d0) at 0x55856a4e9630
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x55856a486840 "\n"\0
  CUR = 1
  LEN = 16
[download]

So only the lines with pragmas are broken. But this wasn't always the case. They used to be broken in a different way. Here with 5.34, we see that pragmas were once breakable.

  DB<2> Dump( ${"_<a.pl"}[6] );
SV = PVMG(0x55874ddaf680) at 0x55874db508a8
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 94039612799016
  NV = 0
  PV = 0x55874db739a0 "use v5.10;\n"\0
  CUR = 11
  LEN = 16
[download]

I'm guessing that when they fixed the bug marking pragmas as breakable, they forgot to set the IOK flag.

Bug report

Fixed in (upcoming) 5.40.

You don't show enough for us to answer. I tried the following in two different versions of Perl (one of them older and one newer than 5.36).

I saved the following script as 2.pl:

#!/usr/bin/perl
use strict;
use feature qw{ say };
use warnings;

my $x = 10;
for my $i (1 .. 10) {
    $x += $i;
}
[download]

I then ran

perl -d 2.pl
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5.26.1:

  DB<1> @lines=@{"_<2.pl"} ; for my $i (0 .. $#lines) { CORE::say $lin
+es[$i] == 0 }
1
1



1



1
[download]

5.39.4 (blead):

  DB<1> @lines=@{"_<2.pl"} ; for my $i (0 .. $#lines) { CORE::say $lin
+es[$i] == 0 }
1
1
1
1
1
1



1
[download]

The output is different, but there's no warning.

You don't get any warnings because you forgot to enable warnings.

I get no warning out of Perl 5.36.1 from the following script:

#!/usr/bin/env perl

use strict;
use warnings;

use v5.36;

say 'Hello, world!';

using the commands:

perl -d dbg.pl
  DB<1> @lines = @{"_<dbg.pl"}; say $_ == 0 for @lines;
1
1
1
1
1
1
1
1


  DB<2> q                                                                       

I also tried 5.36.0, with the same output and the same lack of a warning.

Maybe if you exhibited a small, self-contained example, giving the exact code and exact debugger commands to trigger the problem?

You don't get any warnings because you forgot to enable warnings.

You don't get any warnings because you forgot to enable warnings.

I see use warnings;

Is this not enabling warnings?