It will probably help you to read perlref carefully, and maybe even http://blob.perl.org/tpc/1998/Perl_Language_and_Modules/Perl%20Illustrated/ and perlguts, though the latter is a bit difficult.

The lines

    no strict 'refs';
    foreach my $F (@$Fields) {*{$F} = \ $_t->{$F}->[$_r]};

[download]

alias global variables in package main. After this, those variables are aliases to the elements of the table data structure. When you restore the saved values, you are updating the elements of the table data structure, via the aliases. This is why you see the last row of your table modified (and I am guessing not only the last row, but I didn't run your code).

What you need to do (I think - but test carefully) is make aliases to the original scalars and restore them to the globs (again, see perlref for what this means).

Here is an example of how you might do this:

use strict;
use warnings;
use Devel::Peek;


our ($a, $b, $c);

my @fields = qw(a b);

print "\nBefore setting a value for \$a:\n";
Dump($a);

$a = 'Test';
$b = 'Unused';

print "\nAfter setting a value for \$a:\n";
Dump($a);

$c = 'Clobbered';

print "\nAnd \$c is:\n";
Dump($c);

print "\nGlobref \*a is:\n";
Dump(*a);

{
    no strict 'refs';

    my %tmp;
    $tmp{$_} = \${$_} for(@fields);

    print "\nAfter saving references \\\%tmp is:\n";
    Dump(\%tmp);


    *{'a'} = \$c;

    print "\nAfter aliasing \$a to \$c, \$a is:\n";
    Dump($a);

    print "\nAfter aliasing \$a to \$c, \*a is:\n";
    Dump(*a);

    *{$_} = $tmp{$_} for(@fields);

    print "\nAfter restoring \*a, \*a is:\n";
    Dump(*a);
}

print "\nFinally, \$a is:\n";
Dump($a);

[download]

Which produces:

Before setting a value for $a:
SV = NULL(0x0) at 0x8ecfa60
  REFCNT = 1
  FLAGS = ()

After setting a value for $a:
SV = PV(0x8ebe0c0) at 0x8ecfa60
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x8ed8618 "Test"\0
  CUR = 4
  LEN = 8

And $c is:
SV = PV(0x8ebe0d0) at 0x8edf228
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x8ed4b88 "Clobbered"\0
  CUR = 9
  LEN = 12

Globref *a is:
SV = PVGV(0x8ef2840) at 0x8ecfa50
  REFCNT = 10
  FLAGS = (MULTI)
  NAME = "a"
  NAMELEN = 1
  GvSTASH = 0x8ec0048    "main"
  GP = 0x8ed8c28
    SV = 0x8ecfa60
    REFCNT = 1
    IO = 0x0
    FORM = 0x0  
    AV = 0x0
    HV = 0x0
    CV = 0x0
    CVGEN = 0x0
    LINE = 8
    FILE = "./test.pl"
    FLAGS = 0x2
    EGV = 0x8ecfa50    "a"

After saving references \%tmp is:
SV = IV(0x8ec0e14) at 0x8ec0e18
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x8f01bf8
  SV = PVHV(0x8ec5454) at 0x8f01bf8
    REFCNT = 2
    FLAGS = (PADMY,SHAREKEYS)
    ARRAY = 0x8ecf330  (0:6, 1:2)
    hash quality = 125.0%
    KEYS = 2
    FILL = 2
    MAX = 7
    RITER = -1
    EITER = 0x0
    Elt "a" HASH = 0xca2e9442
    SV = IV(0x8ec0e24) at 0x8ec0e28
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x8ecfa60
      SV = PV(0x8ebe0c0) at 0x8ecfa60
        REFCNT = 2
        FLAGS = (POK,pPOK)
        PV = 0x8ed8618 "Test"\0
        CUR = 4
        LEN = 8
    Elt "b" HASH = 0xdb819b
    SV = IV(0x8ec0e44) at 0x8ec0e48
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x8ecfa90
      SV = PV(0x8ebe0c8) at 0x8ecfa90
        REFCNT = 2
        FLAGS = (POK,pPOK)
        PV = 0x8ee98b8 "Unused"\0
        CUR = 6
        LEN = 8

After aliasing $a to $c, $a is:
SV = PV(0x8ebe0d0) at 0x8edf228
  REFCNT = 2
  FLAGS = (POK,pPOK)
  PV = 0x8ed4b88 "Clobbered"\0
  CUR = 9
  LEN = 12

After aliasing $a to $c, *a is:
SV = PVGV(0x8ef2840) at 0x8ecfa50
  REFCNT = 10
  FLAGS = (MULTI)
  NAME = "a"
  NAMELEN = 1
  GvSTASH = 0x8ec0048    "main"
  GP = 0x8ed8c28
    SV = 0x8edf228
    REFCNT = 1
    IO = 0x0
    FORM = 0x0  
    AV = 0x0
    HV = 0x0
    CV = 0x0
    CVGEN = 0x0
    LINE = 8
    FILE = "./test.pl"
    FLAGS = 0x2
    EGV = 0x8ecfa50    "a"

After restoring *a, *a is:
SV = PVGV(0x8ef2840) at 0x8ecfa50
  REFCNT = 10
  FLAGS = (MULTI)
  NAME = "a"
  NAMELEN = 1
  GvSTASH = 0x8ec0048    "main"
  GP = 0x8ed8c28
    SV = 0x8ecfa60
    REFCNT = 1
    IO = 0x0
    FORM = 0x0  
    AV = 0x0
    HV = 0x0
    CV = 0x0
    CVGEN = 0x0
    LINE = 8
    FILE = "./test.pl"
    FLAGS = 0x2
    EGV = 0x8ecfa50    "a"

Finally, $a is:
SV = PV(0x8ebe0c0) at 0x8ecfa60
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x8ed8618 "Test"\0
  CUR = 4
  LEN = 8

[download]

Have a read (and a play with) Devel::Peek, which I recommend to you as an excellent tool to test your understanding of globs and references, then work your way through the output of the test program. You will see that in the end $a refers not only to a string with the same content, but to the actual original string. This would be important if the originals had magic (tied variables, for example).

In reply to Re: Symbol table manipulation problem by ig
in thread Symbol table manipulation problem by arthurg

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