$re = qr[
    (
        the(?{$rep='The'})
    |
        quick(?{$rep='Quick'})
    |
        brown(?{$rep='Brown'})
    |
        fox(?{$rep='Fox'})
    )
]x;

$s = 'the quick brown fox';

$s =~ s[$re][$rep]g;

print $s;

The Quick Brown Fox