My mistake, I realised how to solve it 
local @ARGV = ('file1.txt');
local $^I = '.bak';
while (<>) {
  m/^(\w+)\s+(\d+)$/;
  printf "$1\t%d\n", exists $fed{$1} ? 0 : $2+1;
}

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In reply to Re^4: update values in one file from another by Anonymous Monk
in thread update values in one file from another by Anonymous Monk

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