and then undef $page->{html}. Shouldn't "undef" be enough?

That depends upon what that hash entry actually is. If it is a reference (or something containing a reference) to data which is held elsewhere, you just clear that slot from the reference, but not the referenced data.

Consider:

perl -le '$foo = "bar"; $h->{foo} = \$foo; undef $h->{foo}; print "foo
+ :$foo:"'
foo :bar:
[download]

which is different to

perl -le '$foo = "bar"; $h->{foo} = \$foo; undef ${$h->{foo}}; print "
+foo :$foo:"'
foo ::
[download]

where the referenced data is cleared.

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