#!/usr/bin/perl
use strict;

my $inputfile = shift;
my $withinBlock = 0;

open (IN, "<$inputfile") || die "could not open $inputfile\n";
while (<IN>) {
    if (/head/) {
        $withinBlock = 1;
        print $_;
        if (/tail/) {
            $withinBlock = 0;
            print "\n";
        }
    }
    if ($withinBlock) {
        print $_;
        if (/tail/) {
            $withinBlock = 0;
            print "\n";
        }
    }
}
close (IN);