Your code:

my $sentence = shift;
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will pick up the FIRST argument passed-in.

Your test probably looked something like:

>perl myprogram i amd testing this.
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and it picked-up just the "i".

There are two possible fixes:
1. Add quotes to the argument, like this:

>perl myprogram "i amd testing this."
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2. Programattically capture ALL passd arguments:

my $sentence = join " ",@ARGV;
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If you notice, @ARGV already contains the same information as @words for this case, so split is unnecessary.