In the C-style for-loop

foreach (;<$fh>;) {
    print " $. :$_ \n";
    }
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the initialization and finalization clauses of the loop (the parts before the first semicolon and after the second semicolon, respectively) are empty. The only clause specified is the conditional clause (between the two semicolons). When the  <> or readline operator is evaluated in the scalar context of the conditional, it returns a single line at a time until the file is exhausted, when it returns undef.

This is equivalent to reading  <$fh> in a while-loop.