"second string in each row" means exactly what?

You have to perform two steps:

1. find the beginning of that "second string"
2. change its first char

For step 2, you can use s///, tr///, or substr. Other have already said how.

For step 1, it depends on how you separate strings. If your strings are separated by spaces (no quoting/escaping), I'd write: $line =~ s/^(\S+\s+)0/${1}2/; meaning:

• match
  • ^ from the beginning of the line
  • ( remember the content of this
  • \S+ some non-spaces
  • \s+ some spaces
  • ) end remembering
  • 0 a literal zero
• replace with
  • ${1} what you remembered in the match
  • 2 a literal 2

-- 
        dakkar - Mobilis in mobile