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        ## Calculate a new field's values based on two others
...
        $t->col('PersonID');
        $t->calc(sub{no strict 'vars'; $PersonID = "$Last$First"});

sub calc
{
...
    ## Return scalar column ref unless return context is undef
    return($WantVals ? $Vals : ());
}

## Don't overwrite fields in package main.
my @saveFields;
...
## Restore fields in main
no strict 'refs'; package main;
foreach my $F (@$Fields) { ${$F} = shift @saveFields; };

#!/usr/bin/perl

...