Looks pretty good, but I'll make some suggestions. Instead of "[^\w]" you can use "\W". It means the same thing. Also putting "if (@pmatch)" before your "foreach" is redundant. If there is nothing in the array, it won't enter the loop. Here's my solution:

while (<DATA>) {
    for (split /\W/) { 
    print "$_\n" if /p/i;
    }
}

__DATA__
CPAN stands for comprehensive Perl Archive Network.
^ and $ are used as anchors in a regular expression.
/pattern/ is a pattern match operator.
Perl is very easy to learn.
Enter 'H' or 'h' for help.
[download]

It separates punctuation and other "\W" characters in the split statement. Then it skips any temporary arrays and just prints if the element contains a "p".