Problem

You need to do something to every word in a file, similar to the foreach function of csh.

Solution

Either split each line on whitespace:

while (<>) {
    for $chunk (split) {
        # do something with $chunk
    }
}

Or use the m//g operator to pull out one chunk at a time:

while (<>) {
    while ( /(\w[\w'-]*)/g ) {
        # do something with $1
    }
}

Discussion

Decide what you mean by "word." Sometimes you want anything but whitespace, sometimes you only want program identifiers, and sometimes you want English words. Your definition governs which regular expression to use.

The preceding two approaches work differently. Patterns are used in the first approach to decide what is not a word. In the second, they're used to decide what is a word.

With these techniques, it's easy to make a word frequency counter. Use a hash to store how many times each word has been seen:

# Make a word frequency count
%seen = ();
while (<>) {
    while ( /(\w['\w-]*)/g ) {
        $seen{lc $1}++;
    }
}

# output hash in a descending numeric sort of its values
foreach $word ( sort { $seen{$b} <=> $seen{$a} } keys %seen) {
    printf "%5d %s\n", $seen{$word}, $word;
}

To make the example program count line frequency instead of word frequency, omit the second while loop and do $seen{lc $_}++ instead:

# Line frequency count
%seen = ();
while (<>) {
    $seen{lc $_}++;
}
foreach $line ( sort { $seen{$b} <=> $seen{$a} } keys %seen ) {
    printf "%5d %s", $seen{$line}, $line;
}

Odd things that may need to be considered as words include "M.I.T.", "Micro$oft", "o'clock", "49ers", "street-wise", "and/or", "&", "c/o", "St.", "TschьЯ", and "Niсo". Bear this in mind when you choosing a pattern to match. The last two require you to place a use locale in your program and then employ \w for a word character in the current locale.

See Also

The split function in perlfunc (1) and in Chapter 3 of Programming Perl; Recipe 6.3; Recipe 6.23