Your example uses base 62. Base 64 more efficient (because it's a power of 2), well tested, produces slightly smaller results.

use MIME::Base64;

$num = 123;

$encoded = encode_base64(pack("V", $num), '');
$encoded =~ s/==$//;
print($encoded, $/);  # ewAAAA (always 6 chars)

$encoded .= '==';
$num = unpack("V", decode_base64($encoded));
print($num, $/);  # 123
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(If you only have int16s, lowercase 'v' (in both places) will result in 3 encoded bytes.)

or even

use MIME::Base64;

$num = 123;

$encoded = encode_base64(pack("V", $num), '');
$encoded =~ s/A+==$//;
print($encoded, $/);  # ew

$encoded = substr($encoded.'AAAAAA', 0, 6) . '==';
$num = unpack("V", decode_base64($encoded));
print($num, $/);  # 123
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