IIRC the PBP advises to omit parens when unambiguously calling built-ins, because (and I agree) it removes extra-noise and improves readability. Thus, CL#1 is written as it was and it runs OK.

Then, looking at CL#2, I thought to omit a pair of parens -- see CL#3. I don't see anything becoming ambiguous, but Perl is confused -- see CL#4. And why the uniq imposes numeric context?

Also curious, if I'm explicitly imposing numeric context on sort (CL#5), Perl warns me 6 times, and not 5, as with CL#3.

>perl -lwe "sub x{@_} print sort x( qw( q w e r t y ))"
eqrtwy

>perl -MList::Util=uniq -lwe "print sort( uniq( qw( q w e r t y )))"
eqrtwy

>perl -MList::Util=uniq -lwe "print sort uniq( qw( q w e r t y ))"
Argument "w" isn't numeric in sort at -e line 1.
Argument "r" isn't numeric in sort at -e line 1.
Argument "y" isn't numeric in sort at -e line 1.
Argument "e" isn't numeric in sort at -e line 1.
Argument "t" isn't numeric in sort at -e line 1.
qwerty

>perl -MO=Deparse -MList::Util=uniq -lwe "print sort uniq( qw( q w e r
+ t y ))"
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use List::Util (split(/,/, 'uniq', 0));
print((sort uniq 'q', 'w', 'e', 'r', 't', 'y'));
-e syntax OK

>perl -lwe "print sort {$a <=> $b} qw( q w e r t y )"
Argument "q" isn't numeric in sort at -e line 1.
Argument "w" isn't numeric in sort at -e line 1.
Argument "e" isn't numeric in sort at -e line 1.
Argument "r" isn't numeric in sort at -e line 1.
Argument "t" isn't numeric in sort at -e line 1.
Argument "y" isn't numeric in sort at -e line 1.
qwerty
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