howzabout:

#!/usr/bin/perl -w
use strict;

my @para = qw|this is my paragraph. this is not all that interesting. 
+hey ho|;
my %hash = ();
print "non-unique words: \n";
## the interesting bit ##
map {$hash{$_}++} @para;
for (keys %hash){print "$_ " if($hash{$_}>1)};
#########################
print "\n";
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gives:

$ perl tmp.pl
non-unique words:
is, this,
$
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