Do you have any ideas on the condition in the loop i can put?

There are many ways to be loopy (in Perl) and it just boils down to what you are trying to accomplish. As I tell my students, first identify the problem and tailor the solution to fit.

Here are a few examples:

# 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
}
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#this also loops until a null happens. 
for(;1;){   # see above for rest of approach
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# Iterate 10 times and quit
for(0..9) {
     # rest of your code 
}
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So there ya have it... a few examples of how it is done. You should read up on flow control in Perl in a handy dandy How To Program in Perl type of book grasshopper.