use Data::Dumper;

sub insert {
  my ($ref, $head, @tail) = @_;
   return unless ( @tail );
  if ( @tail and $head ne 'break') { insert( \%{$ref->{$head}}, @tail ) }
  else {
	  # Here $ref gives '{}' value only.
	  # How to get values of previous head values (ie. f1,f2,f3)
	  print "\n==>", Dumper $ref;

	  # More commands . . .
	  #insert( \%{$ref->{f1}}, @tail )
  }
}


my %hash;
while (<DATA>) {
chomp and insert \%hash, split( '/', $_ ) ;
}
print "\nDump\n", Dumper %hash;

__DATA__
f1/f2/f3/f4/break/f1/f2/f5/break/f1/f2/f6