You could use IO::Scalar, or if your perl is 5.8+ then you could take advantage of the ability to open 'filehandles' to scalars e.g

my $str = '';
open my $fh, '>', \$str;

print {$fh} 'this will end up in $str';

print "\$str contains: $str\n";

__output__

$str contains: this will end up in $str
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For more info see. open and IO::Scalar.

update: reworded first sentence to make sense