#!/usr/bin/perl

use strict;
use warnings;

my $string = "Bei der folgenden Razzia im Teamhotel des Rennstalls [Astana] spielten sich skurrile Szenen ab. Immer mehr Gendarmen, darunter auch Polizeiärzte, trafen bei [Astana] ein.";

my $zeichen = '[';

(my $short = $string) =~ s/^.*?(\w+\s\Q$zeichen\E.*)$/$1/;
print $short;

The regular expression:
(?-imsx:^.*?(\w+\s\[.*)$)


matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  .*?                      any character except \n (0 or more times
                           (matching the least amount possible))
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    \w+                      word characters (a-z, A-Z, 0-9, _) (1 or
                             more times (matching the most amount
                             possible))
----------------------------------------------------------------------
    \s                       whitespace (\n, \r, \t, \f, and " ")
----------------------------------------------------------------------
    \[                       '['
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------