If you want to find a single digit followed by a single upper-case letter anywhere on the line so that "33G" or "5AB" do not match you could use look-around assertions. Look behind assertions can't be variable width so here I use an alternation of two, one for beginning of string (it's an anchor so has zero width) and one for a non-digit (width of one).

use strict;
use warnings;

while( <DATA> )
{
    chomp;
    printf q{%16s : }, $_;
    print m{(?x) (?: (?<=\A) | (?<=\D) ) (\d[A-Z]) (?![A-Z])}
       ? qq{Found $1\n}
       : qq{No match\n};
}

__DATA__
2L
bar.2L
bar.ber.bir. 2L
pob33J.slob
bar.ber.bir.2L
foo.3Hbar
jar.8GH
6Ytootle
par.4T.spootle
[download]

The output.

              2L : Found 2L
          bar.2L : Found 2L
 bar.ber.bir. 2L : Found 2L
     pob33J.slob : No match
  bar.ber.bir.2L : Found 2L
       foo.3Hbar : Found 3H
         jar.8GH : No match
        6Ytootle : Found 6Y
  par.4T.spootle : Found 4T
[download]

I hope this is of interest.

Cheers,

JohnGG