m/<x id=(\d*)([[:alpha:]]*)>/ does not match <tagid=...>. If I change the regex to m/<tagid=(\d*)([[:alpha:]]*)>/, it works.

You should only use most of the Variables related to regular expressions like $1 and $2 if the match was successful, in other words:

    if ( $tag =~ m/<tagid=(\d*)([[:alpha:]]*)>/ ) {
      $tags{sprintf("%04d%6s",$1 || 999,$2)} = $tag;
    }
    else {
      warn "Failed to match: $tag"
    }
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