s/[^[:print:]]/_/g;

##</code><code>##

s/[^[:print:]]/$hash{$&}/g;  # Some people dislike $&

##</code><code>##

s/[^[:print:]]/$hash{$&} || '_'/ge;
# or 
# s/[^[:print:]]/exists $hash{$&} ? $hash{$&} : '_'/ge;