perl -ne "$. ==1 and print;" filename.txt
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perl -e "print scalar <>;" filename.txt
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perl -ne "print and last;" filename.txt
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perl -ne "last; END{print}" filename.txt
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perl -ne "die $_;" filename.txt
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perl -pe "$. > 1 and last;" filename.txt
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Some will be more efficient than others.

The point is that if all you need to print is the first line, you should probably terminate reading the file once you've read the first line.