What your code is doing is printing the entire line (substr $_, 0,) up to but excluding the last backslash. Obviously that's not what you want.

What you need is a regex. /\B(-I\S+)/ will deliver your desired bits in $1, ready to be printed. You need a /g modifier to catch all matches and iterate over them via while. print "$1\n" while /\B(-I\S+)/g; ____________
Makeshifts last the longest.