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

foreach $_s (0..$#$Recs)
{
    $_r = $Recs->[$_s];

    ## Bind $FieldName1, $FieldName2, (etc. for each field name in
    ## $Fields) point to address of the current value for that
    ## field in this record.

    no strict 'refs';
    foreach my $F (@$Fields) {*{$F} = \ $_t->{$F}->[$_r]};

    ## Now $Sub may refer to $_r, $_s, $_t, and ${any field name}

    ## Call $Sub and capture return values iff caller wants them
    ($WantVals ? $Vals->[$_r] = &$Sub() : &$Sub());
}

## Restore fields in main
no strict 'refs'; package main;
foreach my $F (@$Fields) { ${$F} = shift @saveFields; };