open my $fh, '<', $file;
my @ary = <$fh>;
print                          # print the list consisting of
  map { $_->[1] }              # all second elements
  sort { $a->[0] <=> $b->[0] } # of a sorted list of anon arrays
  map {                        # constructed via
    my ($d) = />(\d+)/;        # extracting the numbers to be sorted
   [ $d, $_ ]                  # as an anon array of number and line
  } @ary;                      # for all lines of the file
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See A brief tutorial on Perl's native sorting facilities..
See also: open, print, map, sort. For anonymous arrays see References quick reference, the perldata, perlref and perlreftut manual pages.