sub dbconnect {
    socket(my $mysocket, PF_INET, SOCK_STREAM, getprotobyname('tcp'))
        or die("socket: $!");

    fileno($mysocket) >= 100
        or die("No available file descriptors below 100\n");

    # ...
}
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Update: Here's a test that checks if the above works:

use strict;
use warnings;

use Socket;

my @handles;
for (0..9) {
    open($handles[$_], '<', $0) or die;
    print("$_: ", fileno($handles[$_]), "\n");
}

print("--\n");

close($handles[5]);

socket(my $mysocket, PF_INET, SOCK_STREAM, getprotobyname('tcp')) or d
+ie;
print(fileno($mysocket), "\n");
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Works on my system:

0: 3
1: 4
2: 6
3: 8
4: 9
5: 10
6: 11
7: 12
8: 13
9: 14
--
10
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Thanks, but i was wondering if there any method to check if some number is a socket descriptor, instead of working with range of numbers;

Upd:
Your updated part of post is very good method, thanks, it's very helpful! Would be nice if there are some more methods.

There might be, but I don't know it since it's not something one needs to do. Sounds like an XY problem.

huh? My update just checks if socket uses the first available file descriptor. It doesn't check if some number is a socket descriptor. It doesn't even check if some number is any kind of valid file descriptor.

That's like wondering if a certain address contains a file-descriptor. You might be able to make the determination, but why would you want to?

You could try calling  getsockopt( $mysocket, getprotobyname( 'tcp' ), TCP_NODELAY ).

On my OS I get an error that tells me if the file descriptor provided is not a socket:

getsockopt( STDOUT, getprotobyname( 'tcp' ), TCP_NODELAY ) 
    or warn "?$! :  $^E";;
?Unknown error :  An operation was attempted on something that is not 
+a socket
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