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awk '
    function repl(s,f,t,v)
    {
        return substr(s,1,f-1) sprintf("%-*s", t-f+1, v) substr(s,t+1)
    }

    {
        a=repl($0,800,800,"X")
        print a
    }
' infile

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

$ 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

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

perl -npi.bak -e ' s/(123.{796}) /${1}X/' infile