# loop until I get a null input
while(1){
    print "Type in a nubmer: ";
    my $x=<>;
    exit 0 unless $x
    # rest of the code here
}

##</code><code>##

#this also loops until a null happens. 
for(;1;){   # see above for rest of approach 

##</code><code>##

# Iterate 10 times and quit
for(0..9) {
     # rest of your code 
}