If the last thing that happened was an iteration of the do {} while $cond or do {} until $cond pseudo-loop, you get back a code reference (whether the condition turned out true of false).

That sounds really strange, but sadly I can't reproduce it. I tried

perl -e 'print sub {my $x=5; do {print "."} while $x--}->()'
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but the sub just seems to return 0. Can you give an example? (Also, what version of Perl are you using?)