Are you sure the OP is Perl code? Looks like it might be D.
Code (D 2.065.0 for Win32):

import std.stdio;

void main () {

    int[int] a = [ 1:0, 3:2, 5:4, ];

    foreach (k, v;  a) {
        writefln("%d => %d", k, v);
        }

    }
[download]

Output:

C:\@Work\DMD2\learn\PM\nmm_7280>pm1115263_1
1 => 0
3 => 2
5 => 4
[download]

But whether this is Perl, D or something else, I, like others, still don't understand how your desired output of  a[5:0] relates to the given initial data.