How about

my $p = 'This is an anchor <a name="_Toc00123998">and these are anothe
+r<a name=_Toc00123999"><a name=_Toc00124000"> couple.';

$prefixes = '';

while ($p =~ s/(<a name=[^>]*>)//) {
   $prefixes .= $1;
}

$p = "$prefixes$p";
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