open(IN1, "tmp11") || die "Cannot open this file"; 
@lines = <IN1>; 
$i = 0; 

for (@lines) 
  { 
  $lines[$i]=~s/(^| \K)(?!\d)/0/g;; 
  $thislines[$i]=$lines[$i]; 
  print $thislines[$i]; 
  $i++;
  } 

open(OUT, ">tmp12") || die "Cannot open this file";

for $thisline(@thislines)
  { 
  print OUT $thisline;
  } 

close(OUT); 
close(IN1);
1    2            2        5                4
4    4    4            4                3    
        4    4            1                
    1    5    6        4