Why Dumper output is not evaluated correcty? - perl

Question

I try to eval output of Dumper for pretty simple hashref, where two keys have same value (ref to another hash):
#!/usr/bin/env perl
use strict; use warnings;
use Data::Dumper;
my $foo = { data => 1 };
my $boo = {
x => $foo,
y => $foo,
};
my $VAR1;
my $bar = eval( Dumper( $boo ) );
print Dumper( $boo );
print Dumper( $bar );
I expect the $boo and $bar to have same structure, but eval seems not solve inner-ref $VAR1->{'x'} correctly, I hoped last 2 lines to print same string:
$VAR1 = {
'x' => {
'data' => 1
},
'y' => $VAR1->{'x'}
};
But second has x or y undefined (depending which was referenced in literal form):
$VAR1 = {
'x' => {
'data' => 1
},
'y' => undef
};
I tried simple usage part on doc, and it gave fine results with much more complex structure (no strict, yet), but I can' accomplish it with my data with 2 references to same hash.
What am I missing here?

Answer 1

To correctly capture references inside a structure, you need to set the Purity flag (see the Data::Dumper documentation for details).
$Data::Dumper::Purity = 1;
It's not enough, though, as Dumper($boo) will now return
$VAR1 = {
'y' => {
'data' => 1
},
'x' => {}
};
$VAR1->{'x'} = $VAR1->{'y'};
So, you can't just eval this string, you also need to return $VAR1 from it.
To prevent the purity flag interfering with other parts of the code, you can set it locally:
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
my $foo = { data => 1 };
my $boo = {
x => $foo,
y => $foo,
};
my $VAR1;
my $bar = do {
local $Data::Dumper::Purity = 1;
eval Dumper( $boo );
$VAR1
};
print Dumper( $boo );
print Dumper( $bar );