# ------------- short summary

my $p = 'Terrence and Phillip are sweet';
my ($count,$matchchar,$nummatch,$repchar) = qw(0 e 3 1);
my @q = split//,$p;
for(@q) {
  next unless /$matchchar/;
  $count ++;
  $_ = $repchar if  $count > $nummatch;
}
print 'out === ',@q

# ------------- end short summary.

# First you are not using strict which would have pointed
# out the fact that you declare a var $rep but use a var called
# $repchar and forget to declare $i with my.
#
#!/usr/bin/perl -w reg.pl
# my ($p, @q, $matchchar, $nummatch, $rep, $count, $q, $out);
# $p = 'Terrence and Phillip are sweet';
# $count = 0;
# $matchchar = "e";
# $nummatch = 3;
# $repchar = "1";
#
# Bulk my declaration at top kind of looses the scoping value of 
# my as all these vars are effectively global, although it makes 
# no difference in your script I like to keep my 'mys' local so 
# as to speak
# $rep is unused, this is supposed to be $repchar but with no 
# strict you have not allowed perl to tell you.
# I have declared and assigned vars almost all at once here
#
my $p = 'Terrence and Phillip are sweet';
my ($count,$matchchar,$nummatch,$repchar) = qw(0 e 3 1);
#
# @q = split (/(.*?)/, $p);
#
# Your split syntax is an odd way to split on null, how about
#
my @q = split//,$p;
#
# for($i = 0; $i < $#q + 1; $i++) {
#     $_ = $q[$i];
#     $count += 1 if (/$matchchar/);
#     if (/$matchchar/ && ($count > $nummatch)) {
#         $q[$i] = $repchar;
#     }
# }
#
# You can clean this iteration up heaps.
# You could write
# for my $i(0..$#q) {
# which would shorten it down and cure the missing my but there
# is more. All you need is a 
# for(@q) {
# within this loop each element of the @q array is aliased to the
# magical variable $_ If we modify $_ we modify that array element.
# Also your logic can be improved as you test for a match to 
# $matchchar twice wich is unecessary. I use a next unless 
# construct as it makes it obvious what this loop does - if we do
# not /$matchchar/ we do the next iteration, read no further!
# So without further ado, let's just do:
#
for(@q) {
  next unless /$matchchar/;
  $count ++;
  $_ = $repchar if  $count > $nummatch;
}
#
# $out = join ("", @q[0 .. $#q]);
# print "out === $out";
#
# join'',@q is much shorter than join ("", @q[0 .. $#q]) and does
# the same but but this is shorter.
# print "out ===", join'',@q;
# has the same effect as these two lines and skips the unecessary 
# asignment to $out
# As print @foo is the same as print join '', @foo;
# we can shorten this further to:
#
print 'out === ',@q;  
#
# BTW 
# print "@foo";
# is the same as: 
# print join $", @foo; 
# The output record seperator $" is set to ' ' by default, but you 
# can set it to anything you want. So $"='';print "out === @q"; is 
# yet another variation.

# Here is my version of your code, which is remarkably like the 
# code I did not post in the first place as the Camel book says
# when you think you want to chop a string up into substrings
# what you really eant is the \G asertion.

=pod

#!/usr/bin/perl
use strict;
my $p = 'Terrence and Phillip are sweet';
my ($count,$matchchar,$nummatch,$repchar) = qw(0 e 3 1);
my @q = split//,$p;
for(@q) {
  next unless /$matchchar/;
  $count ++;
  $_ = $repchar if  $count > $nummatch;
}
print 'out === ',@q;

  
=cut