my (@a, %h);
$_ = '1:1,2:1,3:2,500:2,505:1';

#234567890#234567890#234567890#234567890#234567890#234567890

# 47 characters ...
%h=map{@@=/[^:]+/g;$a[$#a+1]=$@[0];@@}/[^,]+/g;

# 44 ...
%h=map{@@=/[^:]+/g;push@a,$@[0];@@}/[^,]+/g;

# Based on duff's whimsy above ... 42 characters
%h=@a=/[^:,]+/g;@a=@a[grep!($_%2),0..$#a];

# Further whimsy ... 29 characters
%h=/[^:,]+/g;@a=/([^,:]+):/g;