Hello karlgoethebier,

The problem is that Perl doesn’t recognise zip as a subroutine when it parses the line in question. As toolic says, one way to solve this is to make the subroutine call unambiguous by removing the whitespace between the subroutine name and the opening parenthesis of the argument list.

Another way is to provide a forward declaration:

use warnings;
use strict;
use feature qw(say);

sub zip;

say zip 'ABCDEFGHIJ', 'abcde';

sub zip {
    join "", sort { lc $a cmp lc $b} split "", $_[0] . $_[1];
}
[download]

Output:

23:44 >perl 1303_SoPW.pl
AaBbCcDdEeFGHIJ

23:45 >
[download]

This is documented in the Camel Book (4^th Edition, 2012, p. 960):

• If you have just a NAME and no BLOCK, it’s a predeclaration of that name.... Named declarations are useful because the parser treats a name specially if it knows it’s a user-defined subroutine.... These are sometimes called forward declarations.

Hope that helps,