Firstly, you should avoid using the scalars $a and $b as they are special and are reserved for use with sort. Secondly, you should consider whether you want to remove the line terminator before doing your split; if you do, look at chomp.

Remember that each element you are passing into the sort routine is a hash reference so you have to de-reference to get to the element you want to sort by. This should do what you want.

my @Data = ();
while ( <FH> )
{
    chomp;
    my ( $meth, $list, $count ) = split m{,};
    push @Data, {
       meth  => $meth,
       list  => $list,
       count => $count,
       };
}
@sortedData =
   sort { $a->{ meth } cmp $b->{ meth } }
   @Data;
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I hope this is useful.

Cheers,

JohnGG