Edit to include code from martin_100's question on sorting lines based on column 2 of a space-delimited file:

print map      { $_->[1] }
      sort     { $a->[0] <=> $b->[0] }
      map      { [(split /\s+/)[1], $_] }
      read_file( $ARGV[0] );
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Let's read the code in reverse order.

    read_file( $ARGV[0] );
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Start by reading in the file line by line.

    map { [(split /\s+/)[1], $_] }
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The lines are passed (as $_) to the 'first' map. $_ is implicitly passed to split /\s+/, which splits the line on spaces. (Note: the , $_ has nothing to do with the split!) Since it is wrapped in (...)[1], the split output is treated like a list in which the 2nd element is returned. This, itself, is inside [ ..., $_], which gives you an anonymous array containing the second element from the split ((split /\s+/)[1]) and the entire original line ($_).

    sort { $a->[0] <=> $b->[0] }
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These anonymous arrays are all passed along to the sort. Sort orders the arrays based on the first element of each anonymous array, hence the ->[0].

    map { $_->[1] }
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Now the sorted anonymous arrays containing both the second column of text and the entire line of text are passed to the 'second' map. This map passes only the second element of each array (the full original line) to the print statement, thereby printing the lines sorted based on the second column!