Interesting. Testing it a little further, it appears that it is the combination /me causing the problems. Removing just the 'm' to give

However, I need the match to be multiline. Re-introducing the /m option in a loop also makes things work:

my $MAX = 100;
while( $html =~ m/<\?KEYWORD\s+([^\?^>]+)\??>/m )
{
  my $ssi_html = $self->handle($1);
  $html =~ s/<\?KEYWORD\s+([^\?^>]+)\??>/$ssi_html/m;
  last if $MAX++ > 100; 
}
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