If I understand you correctly, then you could do this:

my $flag;
while ( defined( $_ = <DATA> ) ) {
    if (/^({\w+})/) {
        $flag = $1;
    }
    elsif ( $flag =~ /{IT}/ ) {
        print $_;
    }
    elsif ( $flag =~ /{TAG}/ ) {
        print $_;
    }
    elsif ( $flag =~ /{ID}/ ) {
        print $_;
    }
}
__DATA__
{IT} 
343
1
{DATE}
090104
{TAG}
1lutherridge0104.ART
{IT} 
434
2
{DATE}
090104
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