use Modern::Perl;

sub extract {
    my ($data, $separator, @fields) = @_;
    return join $separator, (split(/$separator/, $data))[@fields];
}

my $input = 'First#second#third#and this is field no. 4#FIVE5five5#all sixes 666#Seventh and last';
my $extraction = extract($input, '#', 1, 3, 4, 6);
say $extraction;