Bash

VAR='name=@col1, zip_cd=@col2, district=@col3, city=@col4'
echo "$VAR" |
while IFS= read -r line
do
   IFS='=,'; set -f; set -- $line
   while case $# in 0 ) break ;; esac
   do
      shift
      printf '%s' "${1}${2+,}"
      shift
      ${1+':'} echo
   done
done

Sed

echo "$VAR" |
sed -ne '
   H;g
   :loop
      s/\(\n\)[^=]*=\([^,]*,\{0,1\}\)/\2\1/
   tloop
   P
'

Perl

echo "$VAR" |
perl -F'/=|,/' -lpe '
   $_ = join ",", grep { $a++ % 2 } @F;
'