# note the lower-casing inline
($fus = lc $fus) =~ s/\W+/_/g; #all spans of 1 or more non-words become 1 '_'
$fus =~ s/^_|_$//g;            #at most one on each end, because of the above

##</code><code>##

for ($string) {
   s/\s+/_/g;    #convert 1+ spaces to a single '_'
   s/'|^_|_$//g; #trim lead/trail _ and remove single quotes
}

##</code><code>##

$string =~ tr/\s'/_/d;