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use File::Temp qw/tempfile/;
use CGI;

...
# ELSE, we're in the child. Run the long-lived command,
# saving the result in $tempname. Do not attempt to 
# send any information directly to the browser.

my $display_id = $q->param('display_id') // my_fatal_error_handler();

# Error if $display_id is undef
...
# to another page/script
close $fh;
unlink $display_id;