I presume you're trying to replace each string with a specific other string, i.e. XXX is always supposed to be replaced by AAA etc.

my $text="The respondent uses the following products XXX, YYYYYYYYY, Z
+ZZZZZZ around the house and they are considering using QQQQQQQ too. T
+hey are particularly impressed with ZZZZZZZ.";
my %replace=( "XXX"       => "AAA",
              "YYYYYYYYY" => "BBBBBB",
              "ZZZZZZZ"   => "CCCCCC",
              "QQQQQQQ"   => "DDDDDDDD");

my $search=join '|',keys %replace;
$text=~s/($search)/$replace{$1}/g;
print $text;
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