Arbitrarily nested structures cant be matched by proper regular expressions. Luckily perls regular expression engine isnt all that regular so it probably is possible to do this. (japhy knows how I'm sure.)

However it would be so much easier to do it with Text::Balanced or Parse::RecDescent that its unlikely anyone would try. Here is a solution using Text::Balanced

use Text::Balanced 'extract_tagged';
use Data::Dumper;

sub get_ifs {
    my $str=shift;
    my $list=shift || [];

    my ($extracted,
        $remainder,
        $prefix,
        $open,
        $inside,
        $close)=extract_tagged($str,"IF","ENDIF","(?s).*?(?=IF)");
    if ($extracted) {
        push @$list,$extracted;
        get_ifs($inside,$list);
        get_ifs($remainder,$list) if $remainder;
    }
    return $list;
}

print Dumper(get_ifs(<<ENDOFIFS));
IF(A) anytext
    IF(B)
        IF(C) anytext ENDIF
        IF(D) anytext ENDIF
    ENDIF
ENDIF
ENDOFIFS
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