On top of my OO confusion, I was getting led further astray by my overlooking the global nature of filehandles, and my mistaken way of trying to store them in a hash. Got there in the end ... by storing a reference to the file handle in a hash

use FileHandle;
my $FH = FileHandle->new(">>$self->{output_file}");
$self->{file_handle} = \$FH;
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and used in other methods like this

my $FH = ${$self->{file_handle}};    
print $FH "some text\n" ;
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Thanks again for all the pointers.