See https://www.perlmonks.org/?node_id=11154956
perldebguts says:
Each array
@{"_<$filename"}holds the lines of $filename for a file
compiled by Perl. [...] Values in this array are magical in numeric
context: they compare equal to zero only if the line is not breakable.
The last bit is not always true since 5.36.
Take the program
#!/usr/bin/perl use strict; use warnings; use v5.10; say 'Hello, world!';
In 5.36 and 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 v5.10;\n"\0
CUR = 11
LEN = 16
Note that lack of IOK for line 6? It should look like line 5.
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
This also means that only the lines with pragmas are broken.
This is a deviation from 5.34. In 5.34, we see
DB<3> 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
(Same goes at least as far back as 5.10.)
So they were breakable in 5.34, but not in 5.36. Sounds like a valid bug fix. But it sounds like the person that fixed this forgot to set IOK when making that change.
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