use File::Find;
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use File::Find;

my $dir = # whatever you want the starting directory to be

find(\&do_something_with_file, $dir);

sub do_something_with_file
{
    #.....
}
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use File::Find;

find(\&print_name_if_dir, ".");

sub print_name_if_dir
{
    print if -d;
}
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Here, the subroutine print_name_if_dir is given as an argument to find. It simply prints the name of the file if it's a directory. Note the peculiar notation... It's customary in Perl not to mention $_, so:

print if -d;
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Is equivalent to:

print $_ if -d $_;
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Both are quite cryptic (but hey, it's Perl), and for clarity the routine could be rewritten as:

sub print_name_if_dir
{
    my $file = $_;

    print $file if -d $file;
}
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Routines in Perl can be anonymous, which is more suitable for such simple tasks, so the whole program may be rewritten as:

use File::Find;

my $dir = # whatever you want the starting directory to be

find(sub {print if -d}, $dir);
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Just 3 lines of code, and we're already doing something useful !

For the more advanced

The internal variable $File::Find:name can be used at any time to report the full path to the file. Consider the following improved version of our little script:

use File::Find;

find(sub {print $File::Find::name if -d}, ".");
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Try running it and compare the results to the previous version. You will notice that it prints the full path to the directory. What happens is the following - Find::File chdirs into each directory it finds in its search, and $_ gets only the short name (w/o path) of the file, while Find::File::name gets the full path. If, for some reason, you don't want it to chdir, you may specify no_chdir as a parameter. Parameters to find are passed as a hash reference:

use File::Find;

find({wanted => sub {print $File::Find::name if -d}
      no_chdir => 1}, ".");
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Note that "wanted" is the key for the file processing routine in this hash.
The results won't differ from the previous version. Here, however, $_ will also be the full path to a file, because find doesn't "dive into" the directories.
Other parameters may be specified (like 'bydepth' if want a depth-first-search), but these are advanced topics. If you're curious, you can look these issues up in the documentation of the module.

Bonus - a useful utility based on File::Find

Ever felt that your quota suffocates you, and couldn't find the unnecessary large files to remove ? Do you find "du" too tedious to use in these cases ? File::Find comes to the rescue. Consider the following script... It takes a starting directory, and prints the 20 largest files found in the tree under this directory - specifying full paths, so you can just cut-n-paste them into "rm":

#!/usr/local/bin/perl -w

($#ARGV == 0) or die "Usage: $0 [directory]\n"; 

use File::Find;
    
find(sub {$size{$File::Find::name} = -s if -f;}, @ARGV);
@sorted = sort {$size{$b} <=> $size{$a}} keys %size;
    
splice @sorted, 20 if @sorted > 20;
    
foreach (@sorted) 
{
    printf "%10d %s\n", $size{$_}, $_;
}
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What goes on here ? find traverses the given directory recursively, taking notice of each file's size in the $size hash table (-s if -f means = get the size if this is a file). Then, it sorts the hash table by size, and prints the 20 largest files. That's it... I use this utility quite a lot to clean space, I hope you find it useful too (and also understand exactly how it works !)

Update:

Thanks to rinceWind for this:
File::Find is cross-platform. It's one of the really handy ways for iterating directory trees on Windows - something Microsoft don't encourage you to do, with their 'hidden files' (File::Find X-rays through Windows hidden files mechanism nicely :-).

With this in mind, though, you must be careful when working with Windows' paths, because slashes there have a different direction. There is a nice tutorial - Paths in Perl, that explains this.

Update 2:

There are some nice continuation replies written to this tutorial - special thanks to Aristotle, who supplied some info for the real advanced use of File::Find.