I just made the code and output clearer and added a question.

> You never pass an array or hash, just references to them. It's not even possible to pass arrays or hashes to subroutines.

Thats a matter of interpretation, the behaviour of  push @arr, "elem" can be reproduce with prototypes sub name (\@@)

First of all, push is not a subroutine. I'll concentrate on your second example, sub name (\@@).

While it could be a matter of interpretation in general, it's unequivocal in this case because we're talking about the value of $_[0]. The \@ prototype causes a reference to the array to be passed to the sub, not an array. $_[0] contains a reference, not an array. Printing \$_[0] would print a reference to a reference to an array, not a reference to an array.

Push is a CORE:: subroutine which you can override, please check perlsub

> Printing \$_[0] would print a reference to a reference to an array, not a reference to a reference.

well the constant gets always at runtime a reallocated space, while the arrayref doesn't!!!

here a code that makes it hopefully clearer.

UPDATE: no it doesn't, passing a ref to a literal scalar works as wanted!!!

use strict;
use warnings;


sub pr {
    print \$_[0]," : ",$_[0],"\t";      # print ref:value of para
}


check(qw/ Scalar 1 /);
check(qw/ Scalarref \1 /);
check(qw/ Array [1,2] /);
check(qw/ Hash  {1,2} /);



sub check {
    my ($_title_,$type) = @_;
    my $_call_ ="pr $type;";
    my $code= <<"__EOC";
    for (1..3) {
    $_call_
        print "\\n\\t"; 
    for (1..3) {
        $_call_
    }
    print "\\n";
    }
__EOC
    print "\n--- $_title_ \n";
    print $code;
    eval $code;
}


__END__

--- Scalar 
    for (1..3) {
    pr 1;
        print "\n\t"; 
    for (1..3) {
        pr 1;
    }
    print "\n";
    }
SCALAR(0x81953b0) : 1    
    SCALAR(0x8190778) : 1    SCALAR(0x8190754) : 1    SCALAR(0x8190778
+) : 1    
SCALAR(0x8190754) : 1    
    SCALAR(0x819076c) : 1    SCALAR(0x8190778) : 1    SCALAR(0x819076c
+) : 1    
SCALAR(0x8190778) : 1    
    SCALAR(0x81953b0) : 1    SCALAR(0x819076c) : 1    SCALAR(0x81953b0
+) : 1    

--- Scalarref 
    for (1..3) {
    pr \1;
        print "\n\t"; 
    for (1..3) {
        pr \1;
    }
    print "\n";
    }
REF(0x8190754) : SCALAR(0x81953ec)    
    REF(0x8190760) : SCALAR(0x8195428)    REF(0x819073c) : SCALAR(0x81
+95428)    REF(0x8190760) : SCALAR(0x8195428)    
REF(0x819073c) : SCALAR(0x81953ec)    
    REF(0x81953b0) : SCALAR(0x8195428)    REF(0x8190760) : SCALAR(0x81
+95428)    REF(0x81953b0) : SCALAR(0x8195428)    
REF(0x8190760) : SCALAR(0x81953ec)    
    REF(0x8190754) : SCALAR(0x8195428)    REF(0x81953b0) : SCALAR(0x81
+95428)    REF(0x8190754) : SCALAR(0x8195428)    

--- Array 
    for (1..3) {
    pr [1,2];
        print "\n\t"; 
    for (1..3) {
        pr [1,2];
    }
    print "\n";
    }
REF(0x8190754) : ARRAY(0x81953c8)    
    REF(0x8190724) : ARRAY(0x819073c)    REF(0x8190724) : ARRAY(0x8190
+73c)    REF(0x8190724) : ARRAY(0x819073c)    
REF(0x8190754) : ARRAY(0x819073c)    
    REF(0x8190724) : ARRAY(0x81953c8)    REF(0x8190724) : ARRAY(0x8195
+3c8)    REF(0x8190724) : ARRAY(0x81953c8)    
REF(0x8190754) : ARRAY(0x81953c8)    
    REF(0x8190724) : ARRAY(0x819073c)    REF(0x8190724) : ARRAY(0x8190
+73c)    REF(0x8190724) : ARRAY(0x819073c)    

--- Hash 
    for (1..3) {
    pr {1,2};
        print "\n\t"; 
    for (1..3) {
        pr {1,2};
    }
    print "\n";
    }
REF(0x81953c8) : HASH(0x8154708)    
    REF(0x8190748) : HASH(0x81953c8)    REF(0x8190748) : HASH(0x81953c
+8)    REF(0x8190748) : HASH(0x81953c8)    
REF(0x8154708) : HASH(0x81953c8)    
    REF(0x8190748) : HASH(0x8154708)    REF(0x8190748) : HASH(0x815470
+8)    REF(0x8190748) : HASH(0x8154708)    
REF(0x81953c8) : HASH(0x8154708)    
    REF(0x8190748) : HASH(0x81953c8)    REF(0x8190748) : HASH(0x81953c
+8)    REF(0x8190748) : HASH(0x81953c8)
[download]

Cheers Rolf

Thats a matter of interpretation, the behaviour of push @arr, "elem" can be reproduce with prototypes sub name (\@@)

The prototype is just syntactic sugar for taking a reference (plus extra behaviour, for example enforcing list context), so independently of what it looks like on the caller side, the callee always sees a reference, never the array itself.

prototypes are not only syntactic sugar, they are essential to syntax

print prototype 'CORE::push'; # \@@
[download]

Cheers Rolf

Well, why the heck do constants get a new allocated place? Doesn't seem to me as if the compiler does optimisation right!

?? You are not supposed to care one way or the other.