$ perl -le'
my @x = qw[
    one
    two
    three
    four
    ];
sub compare { $a cmp $b }
print for sort compare @x;
'
four
one
three
two
$ perl -le'
my @x = qw[
    one
    two
    three
    four
    ];
sub compare { $a cmp $b }
print for sort compare() @x;
'
Array found where operator expected at -e line 9, at end of line
        (Missing operator before ?)
syntax error at -e line 9, near ") @x"
Execution of -e aborted due to compilation errors.