while ( <DATA> ) {
    chomp;
    my ( $lhs, $rhs ) = split /\s+=>\s+/;
    my $ans = "$lhs => " .
        join ",",
            map { /(\d+)-(\d+)/ ? $1 .. $2 : $_ }
                split /,/;
    print "$ans\n"
}
__DATA__
a => 15-19,30,35,120
b => 15-17,30,35,40,
c => 15-18,30,35,120
__END__
a => 15,16,17,18,19,30,35,120
b => 15,16,17,30,35,40
c => 15,16,17,18,30,35,120