$ro->insert(end => join "", map s/^(\d\d)\d*/$1%/r . "\n", @matches[...] );

$ro->insert(end => join "", map { do { (my $s = $_ ) =~ s/^(\d\d)\d*/$1%/ . "\n"; $s }, @matches[...] });

Try:
    $ro->insert(end => join "", map { (my $r = $_) =~ s/^(\d\d)\d*/$1%/;  "$r\n"; } @matches[0 .. $bestcount - 1]);

Update:

... the s///r pragma.

NB: s///r is not a pragma. /r is a modifier (update: available from Perl version 5.14 on) of the s/// operator.