Here is a easy solution:

#!/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);
[download]

I run it with file

bla head gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla tail gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus head bla bla gugus gugus tail
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus head
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
tail gugus gugus
[download]

and it showed

bla head gugus gugus bla bla gugus gugus
bla head gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla tail gugus bla bla gugus gugus

bla bla gugus head bla bla gugus gugus tail

bla bla gugus gugus bla bla gugus gugus head
bla bla gugus gugus bla bla gugus gugus head
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
bla bla gugus gugus bla bla gugus gugus
tail gugus gugus
[download]

it does not work properly if after a tail there is a head on the same line ...
pelagic

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