From the manual you evidentally didn't find (perldoc) :-)

perldoc -f split
       split /PATTERN/,EXPR,LIMIT
       split /PATTERN/,EXPR
       split /PATTERN/
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In your case, you're only passing in a single argument to split. Perl interprets this as /PATTERN/ and implicitly uses $_ as the EXPR. Since $_ hasn't been initialzed yet (hence the warning) nothing really happens (it's trying to split an undefined value on occurences of "Hello World").

What you need to do (besides taking a look at the perldoc entry for split) is use the two-argument usage of split:

my $Phrase = "Hello World";
my @Parts = split(/ /, $Phrase); # split $Phrase on spaces
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-- Brian