use strict;
use warnings;

my $text = join "", <DATA>;
while($text =~ /<!-- Start_of_revision-->(.*?)<!-- End_of_revision-->/sg)
    {
    print $1;
    }
    
__DATA__

<!-- Start_of_revision-->
revision1
<!-- End_of_revision-->

<!-- Start_of_revision-->
revision2
<!-- End_of_revision-->

<!-- Start_of_revision-->
revision3
<!-- End_of_revision-->