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# style: what i like (my latest first two attempts)
perl -e"printf('%4s: %s',++$a,$_)while(<>)" file
...
# style: [craft] (craft doesn't handle 3digits well)
# you can always change the 2 into 3 to handle 3 digit nums
perl -pe"printf'%-2s: ',++$a" file

########## my favorite (never forget perlvar, thanks davorg)
perl -pe "printf'%4u: ',$." file
...
# or to prevent 'overflow'

perl -pe "printf'%4.4s: ',$." file