@a is an array, or potentially a set of values, while $a is a scalar, or single value. In this case, $a is being used to hold a reference to an array, but the reference is still a single value. I think you probably intended to assign the array values in list context, rather than just adding a reference:

@a = ['1','2','3','4'];    ### NO
@a = ('1','2','3','4');    ### YES
print $_ for @a;    ### Prints 1234
print $a[0];        ### Prints 1
     ### Note that you use $ and not @ when referring to a
     ### single item in the array

$a = ['1','2','3','4'];
print $_ for @$a;   ### Prints 1234
print $a->[0];      ### Prints 1
     ### The -> is used when it's a reference to an array
[download]