split on /\s+\d+,\s+/:

while(<DATA>) {
    chomp;
    my ($k, $v) = split / \s+ \d+ , \s+ /x;
    print "key: $k ... value: $v$/";
    # or stuff it in a hash:
    $hash{$k} = $v;
}

__DATA__
73     63, galileo measur us newton basi law motion
75     45, galileo experi bodi roll slope act same forc weight effect 
+make constantli speed up
70     50, show real effect forc chang speed bodi set move previous th
+ought
75     50, meant bodi act forc keep move straight line same speed
65     45, idea first state explicitli newton principia mathematica pu
+blish 1687 known newton first law
75     50, happen bodi forc act given newton second law
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