Your code is overly complicated. $h{$x} can be replaced with just $h. 

>perl -le"%{ $h }; print $h ? 1 : 0"
0

>perl -le"%{ $h } = (); print $h ? 1 : 0"
1

>perl -le"keys %{ $h }; print $h ? 1 : 0"
1

>perl -le"sub {}->( %{ $h } ); print $h ? 1 : 0"
1

[download]

In the first program, the hash isn't required, so it isn't created.
In the subsequent programs, a hash is required, so it is autovivified.

I can't explain why some built-ins don't autovivify the hash, even though it needs to be flattened. 

>perl -le"sub f {}  f( %{ $h } ); print $h ? 1 : 0"
1

>perl -le"print( %{ $h } ); print $h ? 1 : 0"

0

[download]

In reply to Re: Keys() required to autovivify? by ikegami
in thread Keys() required to autovivify? by jrw

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