$#array = -1

I prefer @array = () for far less magic understanding needed.

if ( $_ =~ /\Q$find\E/s ) { ... }

You can leave off the \E. And that $_ =~ would have me puzzled for a minute or two, being very non-idiomatic. Worth leaving off.

my ($final) = ( $data =~ /\G(.*)$/s ) ;

You should add /c and test the match, otherwise you'll mess up the outer loop if the match fails.

substr($str , 0 , 0) = ">>"

I prefer either $str = "$str>>" or $str =~ s/^/>>/ for that. substr gives me gas, sometimes.

my @lyb = (a..z , A..Z , 0..9) ;

That breaks under use strict, and if the begin or end point is one of the magic single-character syntax things like "q" or "s" or "y". Best to quote the begin/end points.