open "|...", IPC::Open3, IPC::Run, IPC::Run3, etc provide means of feeding input to a child process via its STDIN.

Update:

my $stdin;

while (<STDIN>) {
   $stdin .= $_;

   ... process STDIN as usual ...
}

open(my $child, "|child")
   or die("Unable to start child: $!\n");

print $child $stdin;
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