With a recent (i.e. 5.8.0 onward) perl you can simply open a filehandle to a reference to a scalar rather than a filename. E.g. :

my $foo = "";
                                                                      
+          
open BAR, '>', \$foo;
                                                                      
+          
print BAR "Test test";
close BAR;
                                                                      
+          
print $foo;
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/J\