See IPC::Open2. 
#!/usr/bin/perl
use strict;
use warnings;
use IPC::Open2;

my $pid = open2(my $OUT, my $IN, "/bin/sed", "s/h/H/");
print $IN "hallo\n";
close $IN;
print <$OUT>;

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Output: 

Hallo

[download]
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: How to print to STDIN of a cmd and read it's STDOUT by choroba
in thread How to print to STDIN of a cmd and read it's STDOUT by mhearse

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