D:\>perl -le "print unpack 'd', pack 'd', sqrt 2;" 1.4142135623731 ##

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D:\>perl -le "print unpack 'd', pack 'd', sqrt 2;"
1.41421356237309515

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D:\>perl -le "print unpack 'd', pack 'd', sqrt 2;"
1.41421356237309514547462185873883

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D:\>perl -le "print 'WTF' if (unpack 'd', pack 'd', sqrt 2) != 1.4142135623731;"
WTF

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D:\>perl -le "print 'WTF' if (unpack 'd', pack 'd', sqrt 2) != 1.41421356237309515;"
WTF

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D:\>perl -le "print 'WTF' if (unpack 'd', pack 'd', sqrt 2) != 1.41421356237309514547462185873883;"
WTF