$ cat a.pl
open(FILE, '> bla') or die("$!");
select FILE;
$|=1;
select STDOUT;
print FILE "test1\n";
print FILE "test2\n";
sleep 10;
print FILE "test3\n";
close FILE;

$ cat a.sh
#!/bin/sh
rm -f bla bla2
perl a.pl &
sleep 5
test -f && cp bla bla2
sleep 10
echo bla:
cat bla
echo bla2:
cat bla2

$ a.sh
bla:
test1
test2
test3
bla2:
test1
test2