Perl hashref/property confusion

Question

Ignoring the fact that this probably wouldn't happen if one was using strict and warnings, I'd like to know why these two cases differ.

#!/usr/local/perl5/bin/perl

$x[0] = "";
$y[0] = "";

$x[0]->{name} = "SRV";
$y[0]->{name} = "FINAL";
print "$x[0]->{name}, $y[0]->{name}\n";

$x[1]->{name} = "SRV";
$y[1]->{name} = "FINAL";
print "$x[1]->{name}, $y[1]->{name}\n";

Output is:

FINAL, FINAL
SRV, FINAL

Why, when the index is zero, does the y[0]->{name} assignment affect x[0]->{name}, but not when the index is one?

Thanks,

Sean.

Answer 1

That's not the code you actually ran. In the code you presented, $x[0] and $y[0] are references to different hashes, but in the problematic code, $x[0] and $y[0] are references to the same hash. Like in the following code:

my %hash = { name => "SRV" };
$x[0] = \%hash;           # $x[0] is a reference to %hash.
$y[0] = $x[0];            # $y[0] is a reference to %hash.
$y[0]->{name} = "FINAL";  # Changes $hash{name}.

print $x[0]->{name};      # Prints $hash{name}.
print $y[0]->{name};      # Prints $hash{name}.

The above problem can be fixed by changing

$y[0] = $x[0];

to

$y[0] = { %{ $x[0] } };

or

use Storable qw( dclone );
$y[0] = dclone( $x[0] );