Thanks. If its a array ref: when I tried ...

$joined_FS{$_} ||= {( %{$VAR1->{$_}||{}}, @{$VAR2->{@_}||{}} )} for ke
+ys(%$VAR1), keys(@$VAR2);
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I get same error ... Is it possible to join array ref and hash ref please?

You can't call keys on an array so that's not going to do anything useful. There's a slight whiff of an XY problem in that your data in the two variables isn't of the same shape (HoH vs AoHoH); it might help if you step back a notch and figure out why your previous step(s) aren't making the same outputs.

The cake is a lie.
The cake is a lie.
The cake is a lie.

You can't call keys on an array so that's not going to do anything useful.

As of 5.12 (2010), keys @array returns the indicies of the array.

$ perl -wMstrict -le 'my @x=qw/a b c d/; print for keys @x'
0
1
2
3
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But yeah, in this context it's not useful.

Thank you Fletch for the direction.

I agree with Fletch that it seems a bit odd that your two variables don't have the same data structure. $VAR1 is a hash ref (or more precisely a reference to a hash of hashes), but $VAR2 is an array ref (a reference to an array of hashes of hashes), but there is no obvious reason in the data that you show to have an array as the top data structure. Is this data structure generated by your program at some earlier point? The proper fix would probably be to correct the part of the program that generates $VAR2, but you don't show that part.