Is delete idempotent?

Yes for hash elements. It does not even appear to reset the hash's iterator (which I find surprising). 

$ perl -E'%h=(a=>1,b=>1,c=>1); while (my ($k) = each(%h)) { say $k; }'
c
a
b

$ perl -E'%h=(a=>1,b=>1,c=>1); while (my ($k) = each(%h)) { say $k; de
+lete $h{d}; }'
c
a
b

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Yes for array elements. It doesn't extend the array up to the element to delete. 

$ perl -E'my @a = (4,5); say 0+@a; delete $a[10]; say 0+@a;'
2
2

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(Using delete on array element is documented as being deprecated.)

if I try to delete something that either never existed

Not a problem. 

$ perl -E'my %h; delete $h{x}; say "ok";'
ok

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or has already been deleted

Not a problem. 

$ perl -E'my %h = (x=>1); delete $h{x}; delete $h{x}; say "ok";'
ok

[download]

In reply to Re: Delete idempotence by ikegami
in thread Delete idempotence by derekstucki

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