open demo

# will use this to find out whether the command received the input
bash-4.1$ cat iotest.sh
#!/bin/bash
read -t 1 in
out="$RANDOM"
echo "TEST INPUT $in" >&2
echo "TEST OUTPUT $out" >&2
echo "$out"

# with "|" in front, input can be sent to the command, but command's output can not be read
bash-4.1$ perl -e 'open F,"|./iotest.sh";$out=rand;$in=<F>;print F"$out\n";close F;print"PERL INPUT $in\nPERL OUTPUT $out\n"'
TEST INPUT 0.129122755098255
TEST OUTPUT 26687
26687
PERL INPUT
PERL OUTPUT 0.129122755098255

# "|-" mode string is similar with above
bash-4.1$ perl -e 'open F,"|-","./iotest.sh";$out=rand;$in=<F>;print F"$out\n";close F;print"PERL INPUT $in\nPERL OUTPUT $out\n"'
TEST INPUT 0.261993898618133
TEST OUTPUT 27450
27450
PERL INPUT
PERL OUTPUT 0.261993898618133

# with "|" behind, output can be read from the command, but the command can not get input
bash-4.1$ perl -e 'open F,"./iotest.sh|";$out=rand;$in=<F>;print F"$out\n";close F;print"PERL INPUT $in\nPERL OUTPUT $out\n"'
TEST INPUT
TEST OUTPUT 18103
PERL INPUT 18103

PERL OUTPUT 0.571054091750742

# "-|" mode string is similar with above
bash-4.1$ perl -e 'open F,"-|","./iotest.sh";$out=rand;$in=<F>;print F"$out\n";close F;print"PERL INPUT $in\nPERL OUTPUT $out\n"'
TEST INPUT
TEST OUTPUT 23718
PERL INPUT 23718

PERL OUTPUT 0.985680787664936