UPDATE: Pls ignore this post. Thanks to LanX above I now understand what the problem is.

Can you explain what the supposedly special behavior of split ' ' is? I cannot find any documentation saying anything about it. For me this works:

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

my $str = "I want a split pattern";
my $pattern = $ARGV[0] // ' ';

my @pieces = split /$pattern/, $str;
print Dumper \@pieces;
[download]

hdb$ perl split4.pl
$VAR1 = [
          'I',
          'want',
          'a',
          'split',
          'pattern'
        ];
hdb$ perl split4.pl a
$VAR1 = [
          'I w',
          'nt ',
          ' split p',
          'ttern'
        ];
[download]