$ perl -le 'open FOO, "file.txt"; print length $_ for (<FOO>);'

##</code><code>##

$ perl -le 'print "A" x (9000 + $_) for 1..10' > file.txt

$ perl -lpe '$_=length' file.txt
9001
9002
9003
9004
9005
9006
9007
9008
9009
9010