Question

I would like to be able to capture variable assignments from a Perl eval. That is, to determine what variable names have been assigned to within the code and extract their value.

For example if I run:

eval '$foo=42; $bar=3.14;'

The result of the eval is 3.14 (the last value evaluated), but I would also like to be able to determine the names “$foo” and “$bar” and their values (without knowing the names in advance).

I have read up on a couple of ways of inserting variables into the eval block, through Safe and Eval::Context, but not yet any way of extracting them. I am more familiar with Python’s eval/exec which have built in support for this.

Answer 1

Here’s my attempt at fleshing out a solution based on Safe, as suggested by Eric Strom.

package main;
use warnings; use strict;
use Safe;

my $cpt = new Safe;

$cpt->permit_only(qw(sassign lineseq padany const rv2sv leaveeval));
my $name_space = $cpt->root;

my $no_strict = 0;
#
# populate the clean symbol table
#
$cpt->reval('0');
die "safe compartment initialisation error: $@" if $@;
my %symtab_clean = do {no strict 'refs'; %{$name_space.'::'} }; 

my $result = $cpt->reval('$foo=42; $bar=3.14;', $no_strict);

if ($@) {
    warn "eval error: $@";
}
else {
    #
    # symbol table additions
    #
    my %symtab_dirty = do {no strict 'refs'; %{$name_space.'::'} }; 

    my @updated_variables = grep { ! exists $symtab_clean{$_} } (sort keys %symtab_dirty);

    foreach my $variable (@updated_variables) {
        my $value = do{ no strict 'refs'; ${$name_space.'::'.$variable} };
       print "variable $variable was set to: $value\n"
    }
}

Notes:

1. The above allows a minimal restrictve set of saf opcodes. See perl opcodes
2. I’ve chosen to look for differences before and after executing the command