You can try not use look-around assertions at all, for example by splitting it up into two ordinary alternatives:

my $copyright = qr{\b copyright (?:ed)? \b }xi;
my $date      = qr{\b  (?:19|20) \d\d \b }x;

if ($str =~ s/$copyright.{0,200}$date|$date.{0,200}$copyright/) {...}
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You'd have to check with your real data if it's actually faster, but chances are that it will because it doesn't check every string position for a look-around.

If you want to stick with your look-around approach, you can use look-aheads, which support variable width regexes.