I probably won't be the first to suggest this, but this is a perfect use for the tr/// function:

$newstring =~ tr/01/LH/;
[download]

which translates all 0's to L's and 1' to H's.

Your substitutions would also work by using the 'g' global option at the end, which causes it to substitute every match, not just the first one:

$newstring =~ s/0/L/g;
$newstring =~ s/1/H/g;
[download]

I hope this helps.

Impossible Robot