biohisham and ikegami explained cleanly how you can reference the elements in the first "column" one-by-one in a for loop. If you want to join these elements, you can extend that idiom like this (by storing the values in a new array called @column):

use strict;
use warnings;

my @AoA = ( [ 'Jan', 10, 20 ], [ 'Feb', 15, 25 ], [ 'Mar', 30, 33 ] );

my @column;
for my $month_data (@AoA) {
  push @column, $month_data->[0];
}
print join( ' ', @column ), "\n";
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However you can use shorter and cleaner constructs, if you don't need them one-by-one, but all of them at once, as an array for the "column". Try this:

use strict;
use warnings;

my @AoA = ( [ 'Jan', 10, 20 ], [ 'Feb', 15, 25 ], [ 'Mar', 30, 33 ] );

print join( ' ', map {$_->[0]} @AoA ), "\n"; # prints 'Jan Feb Mar'
print join( ' ', map {$_->[1]} @AoA ), "\n"; # prints '10 15 30'
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The map expression above (map {$_->[0]} @AoA) creates and returns a new array by implicitly iterating through your array refs ("lines", $AoA[0], $AoA[1] ...), taking the first element ("column", $AoA[0][0], $AoA[1][0] ...) from each and constructing the new array from them. (But of course, if you think this is too much for once, just stick with the first and easier version.)