So now it's trivial to find out if "marcus" is an average taxpayer (no) or if mary is (yes). You'd have to write a bit more Perl code than I would have to write Prolog code.

Actually, you don't. It translates pretty straightforward:

$foreigner{abdul} = 1;
$foreigner{marcus} = 1;
$spouse{edward} = "sally"; $spouse{sally} = "edward";
$spouse{bill} = "hillary"; $spouse{hillary} = "bill";
$gross_income{hillary} = 100_000;
$gross_income{edward} = 20_000;
$gross_income{bill} = 30_000;
$gross_income{marcus} = 49_999;
$gross_income{mary} = 35_000;

sub average_taxpayer {my $Person = @_ ? shift : $_;
    !$foreigner{$Person}                and
    not ($spouse  = $spouse{$Person}        and
         $income1 = $gross_income{$spouse}  and
         $income1 > 30_000)                 and
    $income2 = $gross_income{$Person}   and
    $income2 < 50_000
}
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You might want to declare some variables to make it strict (that would be extra code). About the only extra code the Perl part has is 2 extra spouse declarations, as hashes aren't symmetric. How does Prolog know that the "spouse" relationship is symmetric?

However, this is the kicker. What if I want a list of all average taxpayers? Well, I know you have to have a gross income to be an average tax payer, so I issue the following query:

And in Perl:

@avg_taxpayers = grep {average_taxpayer} keys %gross_income;
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Now, I know about logic coding, and I'm aware of its merits. No need to convice me. But you've to come with a better example to show you need less code.