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$w1 = open(fw1,"pgm1|");
...
$read_set->add($w1);

my $pid1 = open my $fh1, "pgm1|";
my $pid2 = open my $fh2, "pgm2|";
...
$read_set->add($fh1);
$read_set->add($fh2);
...

close $fh1 or warn "subprocess failed: status=$?\n";