Another (slightly different) way:

c:\@Work\Perl\monks\perlUser345>perl -wMstrict -le
"print
   map  $_->[0],
   sort { $a->[1] <=> $b->[1] }
   map  [ $_, extract($_) ],
   <>
   ;
 ;;
 sub extract {
   $_[0] =~ m{ (\d+) \s* \z }xms or die qq{bad line '$_[0]'};
   return $1;
   }
"  c.txt
some days are bad 0
sunny, sunny, sunny 1
but not as good as yesterday 2
tomorrow will be better 3
but maybe a few showers 4
today is a good day 5
anything can happen 11
[download]

Where file  c.txt is:

today is a good day 5
tomorrow will be better 3
but not as good as yesterday 2
anything can happen 11
sunny, sunny, sunny 1
but maybe a few showers 4
some days are bad 0
[download]