use strict;
use warnings;

my $data = do {local $/; <DATA>};

if ($data =~ /(\s*{ [^{}]* \bfopen\b ((?: (?>[^{}]+) | {(?-1)} )*) })/x) {
	print $1;
}

__DATA__
void fo
{
	........
	{  //first one
		f = fopen(...);
		.......
		if(...)
		{
		}
	}  //mapping one
	........
}