use warnings;
use strict;

my $str = <<TXT;
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.
TXT

$str =~ s/
            <figr\sn="(\d+?)"> # tag including figure number
            (.*?)              # element contents
            <\/figr>           # close tag
        /
            "<FIGIND NUM=\"$1\" ID=\"FG." . # replacement tag
            sprintf("%03d", $1) .           # padded number
            "\">$2<\/FIGIND>"               # remainder of element
        /gmsxe; # Global, multi-line, ignore newline, ignore whitespace, evaluate
print $str;