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use strict;
use warnings;
...
print WRITEHANDLE "Test!\n";
close WRITEHANDLE;
wait;

open IN, "<&", $ARGV[0] or die $!;
binmode IN;
...
    print "line $_";
}
close IN;

open IN, "<&", $ARGV[0] or die $!;
binmode IN;
my $s = <IN>;
print "line: $s";
close IN;

perl main.pl subcommand1.pl

perl main.pl subcommand2.pl

cat subcommand1.pl | perl subcommand1.pl 0