%hash = (
    a => '15-19,30,35,120',
    b => '15-17,30-35,40',
    c => '15-18,30,35,120',
);

for (keys %hash ) {
    ($s = $hash{ $_}) =~ s/-/../g;
    $hash{$_} = join(',',eval $s);
};
print "$_ => $hash{$_}\n" for sort keys %hash;
__END__
a => 15,16,17,18,19,30,35,120
b => 15,16,17,30,31,32,33,34,35,40
c => 15,16,17,18,30,35,120