You could add an inner loop:

{
    $htm = join('',$1,$2,$3,$4);
    while ( my $htm_line = <$FH_IN> ) {
        last if $htm_line =~ m/^\s*<\/DOCUMENT>/i;
        $htm .= $htm_line;
    }
}
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Note that you need to escape the '/' in the DOCUMENT tag.