My favorite way of doing this is to use Filter::Handle. The following snippet that should do what you want (that is, send all STDERR output to an array called @errors).

#!/use/bin/perl

use strict;
use warnings;

use Filter::Handle qw(subs);

my @errors; #global to hold the output of STDERR

sub filter_errors
{
    #sub for Filter::Handle
    #copies everything sent to STDERR to global @errors
    #and then sends it on to STDERR

    local $_ = "@_";
    push @errors, $_;
    $_;
}

#set up the filtering with Filter::Handle
Filter \*STDERR, \&filter_errors;

#test it
print "Hello world\n";
print STDERR "This is my first error\n";
print "Goodbye world\n";
print STDERR "This is my second and final error\n";

UnFilter \*STDERR; #unfilter the output now that we're done

if (@errors)
{
    print "I got the following errors:\n";
    print "   $_" foreach (@errors);
}
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