The following utilizes the (?PARNO) feature introduced in perl 5.10 to create a recursive regex that captures balanced braces.

It could fail if there are braces included within strings in strange ways, but it should work fine for the way most people code:

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
    ........
}
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