The sort function has extra magic to recognise bare sub names without them being parsed as a call, so you can write sort by_date @list_of_hashes where by_date is the name of a sub. I can’t think of any time I’ve used that feature – however:

use strict;
use File::Spec::Functions;
use Test::More tests => 5;

my @file = do {
    opendir my $dh, curdir or die "$!";
    readdir $dh;
};

sub check(&) { not grep $_ eq updir, shift->( @file ) }

;ok check { sort    no_upwards  @_     }
;ok check { sort    no_upwards( @_ )   }
;ok check { sort  ( no_upwards  @_ )   }
;ok check { sort  ( no_upwards( @_ ) ) }
;ok check { sort +( no_upwards  @_ )   }

=begin output

1..5
not ok 1
#   Failed test at t.pl line 13.
not ok 2
#   Failed test at t.pl line 14.
not ok 3
#   Failed test at t.pl line 15.
ok 4
ok 5
# Looks like you failed 3 tests of 5.
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I just lost half an hour tracking this down.

Makeshifts last the longest.