Perl diagnostic "Unescaped left brace in regex"

Error message seen while deleting a subserver (with actual domain names replaced by example names):

Removing from target virtual website ..
.. Apache website failed! : Unescaped left brace in regex is illegal here in regex; marked by <-- HERE in m/^\%{ <-- HERE HTTP_HOST}\s+=(webmail|admin)\ at /usr/share/webmin/virtual-server/ line 3216.

Removing from email domains list ..
.. done

The perl code around line 3216 is:

# Filter out redirect rules
for(my $i=0; $i<@rcond; $i++) {
        if ($rcond[$i] =~ /^\%{HTTP_HOST}\s+=(webmail|admin)\.$d->{'dom'}/) {
                splice(@rcond, $i, 1);
                if ($rrule[$i] =~ /^\^\(\.\*\)\s+(http|https):/) {
                        splice(@rrule, $i, 1);

This was not a fatal error as the deletion did seem to occur normally.

There is no embedded revision number within the /usr/share/webmin/virtual-server/ file. The global revision is Virtualmin 6.06-2 running on Ubuntu 18.04.2.