When you look at the output you get, you'll notice that sort foo(42) never calls foo. This is because Perl sees it as:

> perl -MO=Deparse -e "sub foo($); sort foo(42);"
sort foo 42
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... which means "sort the list with a single element '42' by using foo as the comparison operator".

If you use the "perl4" method of calling the subroutine (via &foo(...)), Perl knows what you really mean, but you could also tell this to Perl by adding a comma after the sort invocation, to tell sort that it gets a parameter list instead of a sort block:

> perl -MO=Deparse -e "sub foo($){44,33}; print for sort( foo(42), );"
sub foo ($) {
    44, 33;
}
;
print $_ foreach (sort foo(42));

> perl -wle "sub foo($){44,33}; print for sort( foo(42), );"
33
44
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