Don't do multiple passes of the "while". In fact, don't do a "while" at all :)

#!/usr/bin/perl -l

# https://perlmonks.org/?node_id=1224264

use strict;
use warnings;

my $line = '<p><a href="test1.htm"> test1</a><br> <a href="test2.htm">
+ test2</a><br> <a href="test3.htm">test3</a><br> <a href="test4.htm">
+ test4</a><br>';

my @replacements = 'xxx0001' .. 'xxx0100';

$line =~ s/<a href=\"\K.*?(?=\.htm\">)/ shift @replacements /gie;

print $line;
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