choroba is not wrong to write here that "... without a capture group in the regex, there's no way to populate $1." But as always with regexes, it's more complicated:

c:\@Work\Perl\monks>perl -wMstrict -le
"$_ = 'This is not an exit';
 ;;
 if (my ($regex) = /This is/g) {
   print qq{matched, captured '$regex'};
   }
"
matched, captured 'This is'
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Due to the influence of the  /g modifier, something is captured, it's just not anything useful | useful in this case. The only purpose the capture serves in the second OPed SSCCE (the one added in an update) would be to produce a perplexing error message if the open operation failed in the
    open ... or die "could not open 'UserA$regex.txt' $!";
statement. See Matching in list context in perlop.