No surprises, I get the wrong results with double-quotes and the correct results with single-quotes; and identical results in each case, whether delimiting with / or |.

$ perl -p -e "s/(.*?)\.(\w\w\w),/iif([month]>,$1.$2,0) as $2/" z.txt
iif([month]>,.,0) as
iif([month]>,.,0) as
iif([month]>,.,0) as

$ perl -p -e "s|(.*?)\.(\w\w\w),|iif([month]>,$1.$2,0) as $2|" z.txt
iif([month]>,.,0) as
iif([month]>,.,0) as
iif([month]>,.,0) as

$ perl -p -e 's|(.*?)\.(\w\w\w),|iif([month]>,$1.$2,0) as $2|' z.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

$ perl -p -e 's/(.*?)\.(\w\w\w),/iif([month]>,$1.$2,0) as $2/' z.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
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