G'day AmnonM,

Welcome to the monastery.

If you decrement the 'n' value, you can do this:

s/(.)\1{$n}/$1 x $m/eg;
[download]

Here's my test:

$ perl -Mstrict -Mwarnings -E '
    sub mod_char_count {
        my ($n, $m, $x) = @{+shift};
        --$n;
        $x =~ s/(.)\1{$n}/$1 x $m/eg;
        return $x;
    }

    my @data = (
        [4, 1, "p8888er9999l"],
        [1, 3, "monks"],
    );

    say mod_char_count($_) for @data;
'
p8er9l
mmmooonnnkkksss
[download]