To further this oddity:

package Exception;
sub TIESCALAR { bless {}, $_[0] }
sub STORE { print "Changing the value from '$@' to '$_[1]' (not really
+)\n" }
sub FETCH { print "$@\n";42 }

package main;
my $t = tie $@, 'Exception' or die "Unable to do what you want\n";
my ($foo, $bar) = (0, 0);
$@ = "hi";
eval { $foo = 42 / $bar };
print "The value is : $@\n";
$t->STORE("taking a look");
print "Value is still: $@, though\n";
$@ = 'blah';
print "Now the value is : $@\n";
__END__
Changing the value from 'hi' to 'hi' (not really)
The value is : Illegal division by zero at - line 10.

Changing the value from 'Illegal division by zero at - line 10.
' to 'taking a look' (not really)
Value is still: Illegal division by zero at - line 10.
, though
Changing the value from 'blah' to 'blah' (not really)
blah
Now the value is : 42

[download]

Strange that it is actually manipulating $@ in some fashion when you assign something to it.

Update: Ah, antirice showed me some neat tricks. The problem seems to be that the eval's capture is putting the POK flag (has valid public pointer value) on $@. So when you check $@, perl sees the POK flag and just prints what's contained in the PV. It also seems that the PV gets updated no matter what and that checking $@ in the STORE and FETCH are just getting the PV that was set. Funny thing is that the POK flag gets turned off again when you assign to $@ (guess it realizes it's magical once again?). Code and results follow:

# perl
package Exception;
sub TIESCALAR { bless {}, $_[0] }
sub STORE {  }
sub FETCH {  }

package main;

use Devel::Peek;

my $t = tie $@, 'Exception' or die "Unable to do what you want\n";
my ($foo, $bar) = (0, 0);
Dump($@);
eval { $foo = 42 / $bar };
Dump($@);
$@ = 'blah';
Dump($@);
__END__
SV = PVMG(0x8135944) at 0x811b12c
  REFCNT = 1
  FLAGS = (GMG,SMG,RMG,pPOK)
  IV = 0
  NV = 0
  PV = 0x8124100 ""\0
  CUR = 0
  LEN = 240
  MAGIC = 0x8127e40
    MG_VIRTUAL = &PL_vtbl_packelem
    MG_TYPE = PERL_MAGIC_tiedscalar(q)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0x81269a4
    SV = RV(0x8140418) at 0x81269a4
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x811b240
      SV = PVHV(0x812670c) at 0x811b240
        REFCNT = 2
        FLAGS = (OBJECT,SHAREKEYS)
        IV = 0
        NV = 0
        STASH = 0x81269ec       "Exception"
        ARRAY = 0x0
        KEYS = 0
        FILL = 0
        MAX = 7
        RITER = -1
        EITER = 0x0
SV = PVMG(0x8135944) at 0x811b12c
  REFCNT = 1
  FLAGS = (GMG,SMG,RMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x8124100 "Illegal division by zero at - line 13.\12"\0
  CUR = 39
  LEN = 240
  MAGIC = 0x8127e40
    MG_VIRTUAL = &PL_vtbl_packelem
    MG_TYPE = PERL_MAGIC_tiedscalar(q)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0x81269a4
    SV = RV(0x8140418) at 0x81269a4
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x811b240
      SV = PVHV(0x812670c) at 0x811b240
        REFCNT = 2
        FLAGS = (OBJECT,SHAREKEYS)
        IV = 0
        NV = 0
        STASH = 0x81269ec       "Exception"
        ARRAY = 0x8127e60
        KEYS = 0
        FILL = 0
        MAX = 7
        RITER = -1
        EITER = 0x0
SV = PVMG(0x8135944) at 0x811b12c
  REFCNT = 1
  FLAGS = (GMG,SMG,RMG,pPOK)
  IV = 0
  NV = 0
  PV = 0x8124100 "blah"\0
  CUR = 4
  LEN = 240
  MAGIC = 0x8127e40
    MG_VIRTUAL = &PL_vtbl_packelem
    MG_TYPE = PERL_MAGIC_tiedscalar(q)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0x81269a4
    SV = RV(0x8140418) at 0x81269a4
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x811b240
      SV = PVHV(0x812670c) at 0x811b240
        REFCNT = 2
        FLAGS = (OBJECT,SHAREKEYS)
        IV = 0
        NV = 0
        STASH = 0x81269ec       "Exception"
        ARRAY = 0x8127e60
        KEYS = 0
        FILL = 0
        MAX = 7
        RITER = -1
        EITER = 0x0

[download]

In reply to Re: Tying $@ - weird behavior by !1
in thread Tying $@ - weird behavior by Limbic~Region

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