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: (
)