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w1
w2
...
[t4 w4]
w5
w6

w1
w2
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w4 t4
w5
w6

use strict;
  my $file = $ARGV[0];
...
  my @file =<IN>;
  close IN;
m/\[(\S+)\s(\S+)\]/ ? print qq[$2 $1\n] :print qq[$_] for @file;

w1
w2
...
[t4 w4]
w5
w6