It should and does work fine in XP's cmd shell.

>perl -p -e "s|(.*?)\.(\w\w\w),|iif([month]>,$1.$2,0) as $2|" temp.txt
iif([month]>,[01-estimate].jan,0) as jan
iif([month]>,[01-estimate].feb,0) as feb
iif([month]>,[01-estimate].mar,0) as mar
iif([month]>,[01-estimate].apr,0) as apr

>perl -p -e "s/(.*?)\.(\w\w\w),/iif([month]>,$1.$2,0) as $2/" temp.txt
iif([month]>,[01-estimate].jan,0) as jan
iif([month]>,[01-estimate].feb,0) as feb
iif([month]>,[01-estimate].mar,0) as mar
iif([month]>,[01-estimate].apr,0) as apr
[download]

Tested with with ActivePerl 5.6.0, 5.6.1, 5.8.0, 5.8.8, 5.10.0