Here's the module I was working on last night before you found Interpolation.pm. It's harsh, but actually gets the job done, although you have to type your variables as either string or expression. I like the way Interpolation.pm uses import() instead of forcing you to tie your variables yourself, so I borrowed that bit.

Update: Having had a few moments to mull over code generated too late at night, I see it is simpler than I was making it--none of that silly data typing is needed....

package EvalScalar;
use Carp;

sub import {
    my $caller_pkg = caller;
    my $my_pkg     = shift;

    for (@_) {
        my $var = $_;
        $var =~ s/^\$//;
        my $temp;
        tie $temp, $my_pkg;
        *{$caller_pkg . '::' . $var} = \$temp;
    }
}

sub unimport {
    my $caller_pkg = caller;
    my $my_pkg     = shift;

    for (@_) {
        my $var = $_;
        $var =~ s/^\$//;
        my $temp;
        my $old_var = *{$caller_pkg . '::' . $var}{SCALAR};
        *{$caller_pkg . '::' . $var} = \$temp;
        untie $$old_var;
    }
}

sub TIESCALAR {
    my $pkg  = shift;
    my $type = shift;

    my $temp = '';
    my $self = \$temp;
    bless $self, $pkg;
    return $self;
}

sub STORE {
    my $self  = shift;
    my $value = shift;

    $value =~ s/\$(\w+)(?!::)/'$' . caller() . '::' . $1/ge;
    $$self = $value;
}

sub FETCH {
    my $self = shift;
    my $value = eval $$self;
    if ($@) {
        carp $@;
        return '';
    }
    else {
        return $value;
    }
}

1;
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and the example usage...

#!/usr/bin/perl -w
use strict;

use EvalScalar qw($a $b $c);

# Only works with globals
use vars qw($person $num);

$num  = 3;

# Single-quote the rhs if you want
# evaluation of the assignment 
# delayed until read-time.

$a = '$num * 2';
$b = '$a + $num';
$c = '"$num, $a, $b"';

for $num (1..5) {
    print "\$a=$a, \$b=$b, \$c=$c\n";
}

$a = '"Dear $person, this sorta works\n"';

for $person qw(Mom Dad BBQ) {
    print $a;
}

$a = '"But unqualified subs will be called in EvalScalar namespace" . 
+foo()';
print $a;
[download]