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open my $oldin, '<&', \*STDIN or die $!;

...

print "waiting for slave process\n";
waitpid $pid, 0 or die $!

use IPC::Open2 qw(open2);

...

print "waiting for slave process\n";
waitpid $pid, 0 or die $!

open my $oldin, '<&', \*STDIN or die $!;
open STDIN, '<&', \*DATA or die $!;
...

__DATA__
hello world! (from DATA)

my $pid = open my $w, '|-', 'perl -ne print' or die $!;
print "pid: $pid\n";
...

print "waiting for slave process\n";
waitpid $pid, 0 or die $!