local $/ = '';  # Paragraph mode

my $item   = "SeaMonkeys";
my $catNum = "SMKY-1978";
my $maxInstancesToReplace = 2;

my $item_re   = qr/^[ ]*ItemID[ ]*:=[ ]*"\Q$item\E"[ ]*$/m;
my $catNum_re = qr/^[ ]*CatalogNumber[ ]*:=[ ]*"\Q$catNum\E"[ ]*$/m;

my $instances = 0;
while (<DATA>) {
    if (  $instances < $maxInstancesToReplace
       && /$item_re/
       && /$catNum_re/
    ) {
        ++$instances;
        # ...
    }
     
    print;
}