w1
w2
w3
[t4 w4]
w5
w6

##</code><code>##

w1
w2
w3
w4 t4
w5
w6

##</code><code>##

use strict;
  my $file = $ARGV[0];
  open IN, $file or die "Can't open";
  my @file =<IN>;
  close IN;
m/\[(\S+)\s(\S+)\]/ ? print qq[$2 $1\n] :print qq[$_] for @file;

##</code><code>##

w1
w2
w3
[t4 w4]
w5
w6