This is not quite the question I thought of in the chatterbox. Use the IO::Scalar module from CPAN.

use IO::Scalar;
my $capture;
tie(*CAPTURE, 'IO::Scalar', \$capture);
select (CAPTURE);
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And now the output goes into $capture. For details I suggest reading up on tie.

BUT this approach has a big gotcha. Any and all output that comes from Perl will be captured. Any and all output that is done in C is unlikely to be. So for instance any print you do goes to $capture. But if you make a system call, not a chance.

The safer approach is to run a second process like this:

local *CAPTURE;
my $cmd = join " ", "perl", "script_name", @args);
open (CAPTURE,  "$cmd |") or die "Cannot run command $cmd: $!";
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And now the STDOUT of the second process can be read and processed by the first. (On Unix you could actually do a fork and have one process read from the other - the Cookbook has an example.)

EDIT
If people are actually reading this I would like to point out a minor detail. Note how I put the command to run into a variable? I make it a point to always do that so that I don't have to synchronize code to get the debugging message to correctly report what failed. Even if it means having a line like:

my $file = "foo.txt";
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A small but significant detail I like to pay attention to. :-)