Reading the last X bytes is easy enough, an if your lines are fixed length this means the last N lines is equivalent

# Generate some test data
perl -e'for(0..9){@a=(A..Z,a..z," ",0..9);print$a[rand($#a+1)]for 1..8
+0;print$/}'>testdata

# Grab the last 243 bytes == last 3 lines (80 chrs + \n)
perl -le 'open F,"testdata";$a=243;seek F,-$a,2;sysread F,$data,$a;pri
+nt$data;'

gaNLTPyYjzh o0SFnuiNw18l9rpJvHXOcctK2gpLIZzODQou qal15OgfCEQsVGaQ3Ojmb
+5h06pYM3x7
M0GgGwIdqqVmz1D5cWrOwC 0bzvp7bChgOtlzf1TE2 yh77hCIYhricePIH0VxiKHdv3Ye
+a3zkN4aKZ6
sPBR0bryMtGTcGUk87B7UQmEpficHPy0rcjuwYTFyIbweuOcnxtyMpIm2BMcWJa30LVpFQ
+5nLnJesQ4a

# check it is true
tail -3 testdata
gaNLTPyYjzh o0SFnuiNw18l9rpJvHXOcctK2gpLIZzODQou qal15OgfCEQsVGaQ3Ojmb
+5h06pYM3x7
M0GgGwIdqqVmz1D5cWrOwC 0bzvp7bChgOtlzf1TE2 yh77hCIYhricePIH0VxiKHdv3Ye
+a3zkN4aKZ6
sPBR0bryMtGTcGUk87B7UQmEpficHPy0rcjuwYTFyIbweuOcnxtyMpIm2BMcWJa30LVpFQ
+5nLnJesQ4a
[download]

update

that seek should be sysseek to bypass buffering.

Cheers,
R.