It is using implicitly both $. and $_. A possible expanded form could be: 
perl -ne 'print $_ if $. >= 60 and $. <= 70' file.txt

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or, expanding further: 
perl -e 'while (<>) {print $_ if $. >= 60 and $. <= 70;}' file.txt

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In reply to Re^3: Reading specific lines in a file by Laurent_R
in thread Reading specific lines in a file by gurpreetsingh13

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