use strict;
use warnings;

$_ = <DATA>;

if (/<figr n="/) {
  s[<figr n="(\d+)">(.*?)</figr>]
   [qq(<FIGIND NUM="$1" ID="FG.) . sprintf('%03d', $1) . qq(">$2</FIGIND>)]eg;
}

print;

__DATA__
Nerve cells come in many shapes and sizes, but they all have a number of identifiable parts. A typical nerve cell is shown in <figr n="1">Figure 1</figr>. Like all other cells in the body, it has a nucleus that contains genetic information.<figr n="2">Figure 2</figr>. The cell is covered by a membrane and is filled with a fluid.