With recent perls you can open a filehandle to a scalar:

my $out = '';
                                                                      
+          
open OUTH, '>', \$out;
                                                                      
+          
my $var = 'world';
format OUTH =
Hello @>>>>>>>>>>>>>>>
      $var
.
write OUTH; 
                                                                      
+          
print $out;
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