You can try this:

#!/usr/bin/perl 

use strict;
use warnings;

$_="four fifth two frog left leaf fred wolf foul"; 
my @F=m/\bf\w*/g; 
print "@F\n";
@F=m/\w*f\b/g; 
print "@F\n";
@F=m/\w+f\w+/g; 
print "@F\n";
[download]

Results in:

four fifth frog fred foul
leaf wolf
fifth left
[download]

Update: the third case gives you all the words containing an "f" in the middle. To have all words containing but not beginning with an "f" use /[^f]\w*f\w*/g

Update #2: this is better /\b[^f\W]\w*f\w*/g