Try this:

use Data::Dumper;
sub foo { return (); }
warn Dumper(sort {$a <=> $b} foo('1000'));
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and you get what you expect.

Or try:

use Data::Dumper;
sub foo { return (); }
warn Dumper(sort(foo('1000')));
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It looks like Perl thinks that foo is the sort function, and 1000 is what is being passed into the sort. Both those solutions disambiguate the situation.