a variable, which contains a number to a list of numbers, such as 1,2,5,6,9,10,41-56

I'm a little unclear on what you mean. Is this variable an array, like the other monks have assumed? Because if it's a string, you'll have to parse it first. Luckily, there is a module that can handle a string such as the one you've shown, Set::IntSpan. The advantage of this module is that if the spans are large, like say 1,2,3-100000, it'll save a lot of memory as compared to an array or a hash. Otherwise, this is just a TIMTOWTDI solution.

use warnings;
use strict;
use Set::IntSpan;
my $set = Set::IntSpan->new("1,2,5,6,9,10,41-56");
for my $x (1,2,3,42,100) {
    print "$x is ", $set->member($x)?"":"NOT ", "in the set\n";
}
__END__
1 is in the set
2 is in the set
3 is NOT in the set
42 is in the set
100 is NOT in the set
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