I expected the previous code to modify the hash only if the key is already there (and with a true value) .... Am I missing something?

Consider 

$ perl -MData::Dump -e " $foo = 1; dd $foo; $foo &&= 5; dd $foo; "
1
5

$ perl -MData::Dump -e " $foo = undef; dd $foo; $foo &&= 5; dd $foo; "
undef
undef

[download]

$foo exists regardless of the value it has. Similarly $foo{bar} exists by the virtue that you try to assign a value to it (l-value context).

Its like writing  $foo{bar} = ( $foo{bar} && 5 );

In reply to Re: &&= vivify keys? by Anonymous Monk
in thread &&= vivify keys? by oha

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