@decoded = decode_json($result);

##</code><code>##

#!/bin/perl

use v5.10;
use strict;
use warnings;

use JSON::XS qw/decode_json/;
use feature qw/say/;

sub main {
    my $result = qx{ i3-msg -t get_workspaces }; # returns [{},{},{}]
    my $decoded = decode_json($result);
    my $current = 0;

    for my $workspace (@$decoded) {

        my $num = $workspace->{'num'};

        if ($workspace->{'focused'}) {
            $current = $num;
            next;
        }

        # Ignore workspaces below the currently focused one.
        if ($current == 0) { next; }

        # Found the gap!
        if ($current + 1 != $num) { last; }

        # Keep increasing the soon-to-be current workspace.
        $current = $num;

    }

    say $current + 1;
}

main();

##</code><code>##

#!/bin/python3

import json
import subprocess

def main():
    result = subprocess.run(
        ['i3-msg', '-t',  'get_workspaces'],
        capture_output=True,
        check=True
    )

    i3_workspaces = json.loads(result.stdout)
    current_workspace = next((x['num'] for x in i3_workspaces if x['focused']))
    last_workspace = i3_workspaces[-1]['num']
    remaining_workspaces = (x['num'] for x in i3_workspaces if x['num'] >= current_workspace)

    for i in range(current_workspace, last_workspace):
        if i != next(remaining_workspaces):
            print(i)
            return

    print(last_workspace + 1)

if __name__ == '__main__':
    main()

##</code><code>##

time $(for i in {1..100}; do ./move_to_next.pl> /dev/null; done;)

Perl:
real    0m1.113s
user    0m0.843s
sys     0m0.266s

Python:
real    0m1.852s
user    0m1.516s
sys     0m0.323s

##</code><code>##

time ./move_to_next.pl > /dev/null

Perl:
real    0m0.231s
user    0m0.116s
sys     0m0.116s

Python:
real    0m0.099s
user    0m0.078s
sys     0m0.018s