Like this? 

use strict;
use warnings;
my $i = 0;
while(<DATA>){
    s/<foo>.*?<\/foo>/"<foo>".++$i."<\/foo>"/e;
    print;
}
__DATA__
<foo>3</foo>
<bar>a</bar>
<foo>14</foo>
<bar>bc</bar>
<foo>159</foo>
<bar>def</bar>

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In reply to Re^3: sequential substitutions by hdb
in thread sequential substitutions by Anonymous Monk

Title:
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