How do i...how to remove underscore from each line using Perl regular expression?

You can't use a transliteration for this because those operate on characters from a list but you want to match the substring _..

So you have to use a regular expression:

$str =~ s/_\././g;

s: Search and replace
pattern _\.: underscore and dot (dot must be escaped)
replace with .: dot (escaping can be omitted here)
flags g: global (replace all occurences)