([A-Za-z])\.[A-Za-z],$1

##</code><code>##

my $file = "regex.txt";
open FILE, "<", $file or die "Can't open $file: $!\n";
	while(my $regex=<FILE>){
		my @rex = split(/,/,$regex);
		chomp($rex[0]); # $rex[0] is set to "([A-Za-z])\.[A-Za-z]" 
		chomp($rex[1]); # $rex[1] is set to "$1" <--not sure how to get ([A-Za-z])group from $rex[0] to be set here.
		$rrow_test =~ s/$rex[0]/$rex[1]/ig;
	} # END while(my $regex=<FILE>)
close FILE;

##</code><code>##

Reference to nonexistent group in regex; marked by <-- HERE in m/\1 <-- HERE / at C:\utilities\regexTester.pl line 105, <FILE> line 2.