That open statement has things the wrong way around. To open a file handle for printing to a pipeline command, it goes like this:

open my $fh, '| command' or die $!;
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Or better yet, use the 3-arg open call -- and while we're at it, let's provide the command as an array, and get the process-ID:

my @command = qw/downstream_process -a option1 -b option2 -etc/;

my pid = open( my $fh, '|-', @command ) or die $!;
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(The form of open call shown by AnonyMonk would be for running a command so that you can read its output, and you wouldn't write to that file handle.)