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$ cat test.file
123andsomeothervalues with a space at position 22
Not 123 but with the space
123 but no spacein the appropriate column

$ perl -npi.bak -e 's/^(123.{18}) /$1X/' test.file

$...

perl -npi.bak -e ' s/(123.{797}) /${1}X/' infile