Hello nvlung, and welcome to the Monastery!

LanX and choroba have answered your question, but a little code may help to make things clearer:

#! perl
use strict;
use warnings;

my $new_msg =
'I have <c> apples, <p> pineapples, and <t> pieces of fruit.
Yes, I do have <c> apples!';

my $count = 7;
my $pcnt  = 5;
my $total = $count + $pcnt;

print "\nBefore applying the substitutions, \$new_msg is:\n$new_msg\n"
+;

$new_msg =~ s/<c>/$count/g;
$new_msg =~ s/<p>/$pcnt/g;
$new_msg =~ s/<t>/$total/g;

print "\nAfter applying the substitutions, \$new_msg is now:\n$new_msg
+\n";
[download]

Output:

18:30 >perl 820_SoPW.pl

Before applying the substitutions, $new_msg is:
I have <c> apples, <p> pineapples, and <t> pieces of fruit.
Yes, I do have <c> apples!

After applying the substitutions, $new_msg is now:
I have 7 apples, 5 pineapples, and 12 pieces of fruit.
Yes, I do have 7 apples!

18:30 >
[download]

Note that the /g modifier on the substitutions “specifies global pattern matching--that is, matching as many times as possible within the string.” (perlop) If you remove the /g from the first substitution, only the first occurrence of <c> will be substituted.

Hope that helps,