use strict;
use warnings;
sub println {
       print((@_ ? join($,, @_) : $_))."\n";
   }
println "hello world";
println "goodbye world";

# output:
Use of uninitialized value in join or string at foo line 4.
Use of uninitialized value in join or string at foo line 4.
hello worldgoodbye world
[download]

Secondly, your implementation is broken

my bad.

sub println {
    $\ = "\n";
    @_ = ($_) unless @_;
    print @_;
}
[download]