expanding on AnomalousMonk's code and haukex's, it does seem like the sig handler has something to do with this. It appears to be activating the NV slot of the scalar.

When perl then does its DWIM conversions, it is preferring the double over the int, so I would percieve that the scalar is something a little more complex than just a dualvar.

Also worth noting sig handling on some operating systems are implemented differently. how much impact this is having I am unsure of

use strict;
use warnings;
use Devel::Peek;
$|++;
my @TEST_VALUES = ('a55','55a');
 foreach my $i (@TEST_VALUES)
 {

   print qq{\nCalling TEST('$i') / \n};
   
   print qq{\nTEST return values = @{[ TEST($i) ]} \n\n};
   
}

 sub TEST
 {
   my $N = shift;

   local $SIG{__WARN__} = sub { $|++; print "in_warn\n";
#    my $subref = sub { $|++; print "in_subref\n";
     Dump $N;
     print qq{(warn: N=$N -> };
     $N = 77;
     print qq{N=$N) / \n};
     Dump $N;
     print "outta_subref\n";
     };

   print "in_test\n";
   Dump $N;
#   my $tmp_value = $subref->();
   print qq{N=$N / N+0=}, $N + 0;
   print qq{ / N.''=}, $N.'', qq{\n};
 
   print "outta_test returning $N, $N+0, $N.''\n";
   return $N, $N+0, $N.'';
 }


=head1 output

Calling TEST('a55') /
in_test
SV = PV(0x10a847c) at 0xf9d3dc
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  PV = 0xf9055c "a55"\0
  CUR = 3
  LEN = 12
in_warn
SV = PVNV(0x10a9bcc) at 0xf9d3dc
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0xf9055c "a55"\0
  CUR = 3
  LEN = 12
(warn: N=a55 -> N=77) /
SV = PVNV(0x10a9bcc) at 0xf9d3dc
  REFCNT = 2
  FLAGS = (PADMY,IOK,POK,pIOK,pPOK)
  IV = 77
  NV = 0
  PV = 0xf9055c "77"\0
  CUR = 2
  LEN = 12
outta_subref
N=a55 / N+0=0 / N.''=77
outta_test returning 77, 77+0, 77.''

TEST return values = 77 0 77


Calling TEST('55a') /
in_test
SV = PVNV(0x10a9bcc) at 0xf9d3dc
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0xf9055c "55a"\0
  CUR = 3
  LEN = 12
in_warn
SV = PVNV(0x10a9bcc) at 0xf9d3dc
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  IV = 0
  NV = 55
  PV = 0xf9055c "55a"\0
  CUR = 3
  LEN = 12
(warn: N=55a -> N=77) /
SV = PVNV(0x10a9bcc) at 0xf9d3dc
  REFCNT = 2
  FLAGS = (PADMY,IOK,POK,pIOK,pPOK)
  IV = 77
  NV = 55
  PV = 0xf9055c "77"\0
  CUR = 2
  LEN = 12
outta_subref
N=55a / N+0=55 / N.''=77
outta_test returning 77, 77+0, 77.''

TEST return values = 77 55 77

=cut
[download]