First of all: ++. Then (I wanted to /msg you, but it resulted to be longer than expected, so:)

$ perl -le 'open FOO, "file.txt"; print length $_ for (<FOO>);'
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Ok, they're just ten lines so it doesn't make a difference, but we recommend people all the time not to slurp files in all at once if possible and since we're talking about oneliners anyway, I would rewrite them like thus:

$ 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
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