-p uses it to print the output at the end of every loop.

>perl -MO=Deparse -pe1
LINE: while (defined($_ = <ARGV>)) {
    '???';
}
continue {
    print $_;
}
-e syntax OK
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Another example is C-style for loops.
for ($i=0; $i<10; ++$i) { foo }
can be written as
$i=0; while ($i<10) { foo } continue { ++$i }