It's easy and safe with threads.

#! perl -slw
use strict;
use threads qw[ async ];
use threads::shared;

$| = 1;

## Simulate tail
open FILE, q[
    perl -le"$|=1; print and select(undef,undef,undef,.1) for 1 .. 30"
+ |
] or die $!;

my $done : shared  = 0;
my $t = async {
    while( !$done and sleep 1 ) {
        print 'Blah';
    }
};

while( <FILE> ) {
    printf;
}

$done = 1;
$t->join;

__END__
P:\test>397046
1
2
3
4
5
6
7
8
9
10
Blah
11
12
13
14
15
16
17
18
19
Blah
20
21
22
23
24
25
26
27
28
Blah
29
30
Blah

P:\test>
[download]