IO::Scalar might work for you -- this is a snippet from its man page:

use IO::Scalar;
my $s;
tie *STDOUT, 'IO::Scalar', \$s;
print "line 1\nline 2\n", "line 3\n";
warn "String is now: $s\n"
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Also take a look at this Q&A node for IO::Handle solutions: how do i redirect STDOUT, STDIN, or STDERR to a FILE?