Another important change is to control flow logic and code organization

open ... or die Error(...);

open ... or return ErrorPage(...);

If you can't open the file, don't try to read from it or write to it, end the subroutine (the code should be in subroutines

Don't generate your pages by interleaving print statements, make subroutines, so your program is

Main( @ARGV );
exit( 0 );
sub Main {
    my $q = CGI->new;
    print SomeKindOfPages( $q );
}
sub SomeKindOfPages {
    my( $q ) = @_;
    ...
    my( $headers, $body ) = ShippingEstimator( $q , ... );
    return $headers, $body;
}
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