This assumes your markers are the only text on the line (or at least the only text you care about). You'll have to adjust and do some extra 's///' if that's not the case:

use File::Temp qw(tempfile);
use Cwd;

sub replace_file {
 my ($file, $start, $end, $replace_text) = @_;
 local (*ARGV, $_, $.); @ARGV = $file;
 my ($fh, $tmp_file) = tempfile(DIR=>cwd);
 my $replaced;
 while (<>) {
   my $replace = /\Q$start\E/../\Q$end\E/;
   print $fh $_ and next unless $replace;
   $replaced=1;
   print $fh $replace_text if $replace == 1;
 }
 close $fh;
 return unless $replaced;
 rename $tmp_file, $file or warn "Error replacing $file: $!";
}
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Update: upon closer inspection, I see my answer is similar to BrowserUK's above, though my answer also deals with actually replacing the file, not just outputting the replaced text, and should work within a File::Find solution.