#!/usr/bin/perl

$first = "Four Score and Seven";
$last = "thank you and now go to hell.";

$para = <<'EOF';
Blah blah
and stuff.
EOF

open(IN, "<file.txt");
$file = join('', <IN>);
close(IN);

$file =~ s/$first.*$last/$para/sm;

open(OUT, ">file.txt.new");
print OUT $file;
close(OUT);