Hello $h4X4_|=73}{,

These lines look strange to me:

if (exists $xlist{$name} && defined $xlist{$name}) {
 $xlist{$name} = undef; # this exists now
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However, if the logic is correct, you can gain a little speedup by removing the first test:

if (defined $xlist{$name}) {
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Testing for definedness on a non-existent hash entry doesn’t raise a warning, and won’t trigger autovivification, as the following demonstrates:

22:44 >perl -Mstrict -MData::Dump -wE "my %h = (a => 1, b => undef); d
+d \%h; say $_, defined $h{$_} ? ': yes' : ': no' for qw(a b c); dd \%
+h;"
{ a => 1, b => undef }
a: yes
b: no
c: no
{ a => 1, b => undef }

22:47 >
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Hope that helps,