On Perl 5.8 or newer you can do:
use strict;

my $err_string;

close STDERR;
open STDERR, '>', \$err_string or die "Could not redirect STDERR: $!";

print STDERR 'This is an error!';

print "***$err_string***\n";

[download]
See perldoc -f open for the details.

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

In reply to Re: Capturing STDERR to a variable by CountZero
in thread Capturing STDERR to a variable by oron

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