Ah, Juerd's comment about return explains why I was unable to replicate your problem with any version of perl I have. I automatically corrected it when writing my test code.

Anyways, in case someone is interested, here's the test code. I've modified it to show the problem with return; it won't work on Windows as is, you'll have to remove the 2>/dev/null.

#!/usr/bin/perl -w

use strict;


{
    my $const_return   = 'BEGIN { eval "sub HAS () { return 1 }" }';
    my $const_noreturn = 'BEGIN { eval "sub HAS () {        1 }" }';
    my $const_use      = 'use constant; BEGIN { constant->import( HAS 
+=> 1 ) }';

    my $paren   = 'print HAS;';
    my $noparen = 'print HAS();';

    print(
        'return, with parens       ', run($const_return,   $paren  ), 
+"\n",
        'return, without parens    ', run($const_return,   $noparen), 
+"\n",
        'noreturn, with parens     ', run($const_noreturn, $paren  ), 
+"\n",
        'noreturn, without parens  ', run($const_noreturn, $noparen), 
+"\n",
        'use, with parens          ', run($const_use,      $paren  ), 
+"\n",
        'use, without parens       ', run($const_use,      $noparen), 
+"\n",
    );
}



sub run {
    my($code) = join ' ', @_;

    my @normal  = qx{perl             -wle \Q$code\E            };
    my @deparse = qx{perl -MO=Deparse -wle \Q$code\E 2>/dev/null};

    chomp(my $normal_output  =  $normal[-1]);
    chomp(my $deparse_output = $deparse[-1]);

    return join '  ', $normal_output, $deparse_output;
}
[download]

And some example output, with 5.12.2:

> px 5.12.2 ./constant-folding.pl 
return, with parens       1  print HAS;
return, without parens    1  print HAS;
noreturn, with parens     1  print 1;
noreturn, without parens  1  print 1;
use, with parens          1  print 1;
use, without parens       1  print 1;
[download]

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