http://www.perlmonks.org?node_id=451791

Sprad has asked for the wisdom of the Perl Monks concerning the following question:

Observing a hash changes its contents...

I'm using a hash of hashes, and doing an existence check. If the check fails, I print a usage giving the valid keys. Strangely, the invalid key is being created by the existence check. I added a line to delete it, which fixes it, but why is this happening at all?

Here's a sample repro:

#!/usr/bin/perl -w use strict; my %hash = ('a' => { 'value' => 1, 'foo' => 'bar', }, 'b' => { 'value' => 2, 'foo' => 'bar', }, 'c' => { 'value' => 3, 'foo' => 'bar', }, ); if (! $hash{'d'}->{'value'}) { print "That key doesn't exist. Valid keys are: "; print join (", ", keys %hash); }
'd' appears in the output list. No change if I use defined() for the exist check.

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