my $split_pattern = ' '; # default
...
# $split_pattern might get changed here from a command line option
$split_pattern = $foo if $bar;
...
$line = "   one two three   \n";
my @words = split $split_pattern, $line;
for my $i (0..$#words) {
    print "$i: ($words[$i])\n";
}

# Expected output for default case
0: one
1: two
2: three

# Actual output for default case
0: ()
1: ()
2: ()
3: (one)
4: (two)
5: (three)
6: ()
7: ()
8: (
)
[download]

Now, I understand what you are looking for, it wasn't clear to me so far. If you store ' ' into a variable, you don't get the expected behavior as if you hard code:

my @fields = split ' ', $line;

Using "qr/\s+/;" as a split pattern improves the result but still does not yields what you want (you still get an extra empty element at the beginning of the array).

Well, then I don't see any real simple direct solution. Even something like this:

my @fields = split $pattern? $pattern : ' ', $line;

does not do what you are looking for. The only solutions I can think of are either to use the above "qr/\s+/;" and shift the first element if empty, or use a if else construct, or yet this equivalent construct:

my @fields  = defined $pattern? split $pattern, $line : split ' ', $line;

Many thanks. I had not seen that bit.