perl -le "$str='a\\:b:c'; my (undef,$x,$y)=split /((?:[^:\\]+|\\.)*):/
+s,$str,2; print for $x,$y"
a\:b
c
[download]

perl -le "$str='a\\:b:c'; my ($x,$y)=$str=~/^((?:[^:\\]+|\\.)*):(.*)$/
+s; print for $x,$y;"
a\:b
c
[download]