Check out IO::Scalar. It can be used like this:

perl -MIO::Scalar -le 'my $s; $SH = IO::Scalar->new(\$s); print $SH "f
+oo"; print $s'
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Update: If you need to, you can even alias STDOUT to your IO::Scalar handle:

use IO::Scalar;

my $s;
my $SH = IO::Scalar->new(\$s);

{
    local *STDOUT = $SH;
    print "foo\n";
}

print $s;
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That technique should enable you to avoid changing the current code.

Update 2: A better way to do that, rather than aliasing, is to use the tie interface provided by IO::Scalar. I probably should have recalled that, as I answered a similar question just over a month ago.

use IO::Scalar;
$string = tie *STDOUT, 'IO::Scalar';
print "foo\n"; 
print STDERR '$string: ', $string;    # Must use stderr as stdout is t
+ied.
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-sauoq
"My two cents aren't worth a dime.";