As others have pointed out, your problem is you are asking Perl to dereference a reference to a variable that has not been defined.

This demonstrates one of the benefits of using full deference syntax rather than omitting the optional braces:

%{ $href } # better than %$href
@{ $aref } # better than @$aref
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Why better ? Besides being more readable, the full syntax allows you to put expressions in the dereference, so that in your case you could do:

if ( scalar @{ $lib || [] } ) {
    ...
}
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Including this because I did just get bitten by it recently:
Also, be cautious of using scalar to test whether an array is populated. If you need to know whether there are non-empty values in the array, scalar can trip you up.

$ perl -Mstrict -E 'my $aref = [ "", undef ]; say scalar @{ $aref || [
+] } ? "True" : "False";'
True
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If you want to count only non-empty elements in your array, filter for them using grep:

$ perl -Mstrict -E 'my $aref = [ "", undef ]; say scalar ( grep { leng
+th $_ } @{ $aref || [] } ) ? "True" : "False";'
False
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Hope this helps!