#!/usr/bin/perl

use strict;
use warnings;

open my $cmd, "-|", "(SHELL_COMMAND | sed 's/^/stdout:/') 2>&1";

my $stdout;
my $stderr;

while( <$cmd> ) {
  if( s/^stdout:// ) {
     $stdout .= $_;
  } else {
     $stderr .= $_;
  }
}
close $cmd;

print "This is stdout:\n$stdout";
print "This is stderr:\n$stderr";