You can try redirecting STDERR to a file and then reading back from that file, for the duration of $somecommand. Perhaps something like:
use Fcntl qw(:seek);

# save STDERR filehandle
open(my $ORIGSTDERR, ">&", STDERR);

# create temporary filehandle to catch STDERR
open(my $CATCHERR, "+>", undef) or die("cannot open temp file :: $!");
open(STDERR, ">&", $CATCHERR); # redirection


## STDERR is captured here ##
open(PIPE, '-|', $somecommand, $arg1, $arg2, $argN, $filename);

...


# restore STDERR filehandle
open(STDERR, ">&", $ORIGSTDERR);

# read captured STDERR from temporary filehandle
seek($CATCHERR, 0, SEEK_SET);
print "From CATCHERR: $_" while <$CATCHERR>;
close($CATCHERR);

[download]

Using a module I wrote (Pipe processes and Perl subroutines together), the STDERR capture can be further localized for the child process only:

use IPC::Exe;  ## exports exe() & bg()

use Fcntl qw(:seek);

# create temporary filehandle to catch STDERR
open(my $CATCHERR, "+>", undef) or die("cannot open temp file :: $!");

# $cmd is a CODE reference to exec $somecommand
#my $cmd = exe sub { open(STDERR, ">&", $CATCHERR) }, $somecommand, @a
+rgs, $filename;

my $cmd = exe sub { open(STDERR, ">&", $CATCHERR) }, 'perl', '-le', 'p
+rint "From STDOUT"; print STDERR "From STDERR"; exit 33';

my @pid_and_err = $cmd->();     # do it!

my $pid = shift(@pid_and_err);  # only one PID
$? = pop(@pid_and_err);         # last array item is $CHILD_ERROR

print "PID was $pid\n"; 
print "Exit status was ", ($? >> 8), "\n";

# read captured STDERR from temporary filehandle
seek($CATCHERR, 0, SEEK_SET);
print "From CATCHERR: $_" while <$CATCHERR>;
close($CATCHERR);

__END__
Output:

From STDOUT
PID was 6914
Exit status was 33
From CATCHERR: From STDERR

[download]

In reply to Re: Pipe, STDERR, STDOUT, and friends by repellent
in thread Pipe, STDERR, STDOUT, and friends by devnul

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