First of all, that's not true. The following are all acceptable:

push @a;
push @a, 'a';
push @a, 'a', 'b';
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Second, if it was true, that doesn't answer why the second parameter can't be optional, defaulting to $_.

push @a;

It is syntax which will compile, but it does generate a warning. Can you show an example of where this would be used?

use strict;
use warnings;
use diagnostics;
use Data::Dumper;

my @a = 1..3;
push @a;
print Dumper(\@a);

__END__
% ./728108.pl
Useless use of push with no values at ./728108.pl line 9 (#1)
    (W syntax) You used the push() or unshift() function with no argum
+ents
    apart from the array, like push(@x) or unshift(@foo). That won't
    usually have any effect on the array, so is completely useless. It
+'s
    possible in principle that push(@tied_array) could have some effec
+t
    if the array is tied to a class which implements a PUSH method. If
+ so,
    you can write it as push(@tied_array,()) to avoid this warning.

$VAR1 = [
          1,
          2,
          3
        ];
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Toolic, we perfectly know it's not implemented, we meditate WHY it's not implemented!

Here a simple usecase:

for (1 .. 100) {
 next if $_%2;
 push @even;
}
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Cheers Rolf

UPDATE...PS: Thanx for the interesting link to Builtin functions defaulting to $_

You wouldn't use such a construct. It just showed that your statement wasn't true. So whatever point you were trying to get across was even more lost to me.