in reply to How do I make all the keys in %a with common keys in %b equal to %b's values?

for (keys %a) {
    $a{$_} = $b{$_} if exists $b{$_};
}

# or 

for (keys %b) {
    $a{$_} = $b{$_} if exists $a{$_};
}

# or

$a{$_} = $b{$_} for grep { exists $b{$_} } keys %a;

# or

$a{$_} = $b{$_} for grep { exists $a{$_} } keys %b;

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(whether to iterate over keys %a or keys %b is essentially arbitrary — pick the one with fewer keys (if known beforehand) )

In Section Seekers of Perl Wisdom