$_ = "
IF(A) anytext
IF(B) anytext ENDIF
IF(C) anytext ENDIF
and on ....
ENDIF
";
@( = ('(','');
@) = (')','');
($re=$_)=~s/((\bIF\b)|(\bENDIF\b)|.)/$([!$2]\Q$1\E$)[!$3]/gs;
print join"\n\n",eval{/$re/},"";
warn $@ if $@=~/unmatched/;