join, like all perl functions, takes lists as arguments (and conveniently provides them in the magical @_ array).

Another good example is substr. It can be used in lvalue context ( special context for subroutines, means you can assign to their result)

use strict;
use warnings;
my $a = 'abcdefg';
print $a,$/;
print substr($a,0,3),$/;       # rvalue (list)
print $a,$/;
print substr($a,0,3)='bob',$/; # lvalue
print $a,$/;
print scalar( substr($a,0,3,'ABC')),$/; # rvalue (scalar)
print $a,$/;
__END__
abcdefg
abc
abcdefg
bob
bobdefg
bob
ABCdefg
[download]

As you can see substr has special behaviour in lvalue context. RValue means the result is treated as an ordinary value.