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;