#!/usr/bin/perl
use strict;
use warnings;

{
    local $/='<tr>';
    while (my $line = <DATA>){
        my $i=0;

        while ($line =~ m/<tc>/) {
            $line =~ s!<tc>!'<tc cNum="'.++$i.'">'!ie;
            #why not insert another substitution in here that handles the case <tc (colspan="\d+")> ...
        }
        print $line;
    }
}

__DATA__
<tr><tc>1</tc><tc>2</tc><tc>3</tc></tr>
<tr><tc colspan="2">1and2</tc><tc>3</tc></tr>
<tr><tc>1</tc><tc colspan="2">2and3</tc></tr>
<tr><tc>1</tc><tc>2</tc><tc>3</tc></tr>