Setup: I made sample.xlsx
with the following sheets (and the same 2x2 table on each sheet):

xlsxfile <- "sample.xlsx"
sheetnames <- readxl::excel_sheets(xlsxfile)
sheetnames
# [1] "a_1" "a_2" "a_3" "b_1" "b_2" "c_1" "c_3"
newxlsxfiles <- sub("_.*", "", sheetnames)
newxlsxfiles
# [1] "a" "a" "a" "b" "b" "c" "c"
dats <- lapply(setNames(nm = sheetnames), readxl::read_xlsx, path = xlsxfile)
str(dats)
# List of 7
# $ a_1: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# ..$ a: num 1
# ..$ b: num 2
# $ a_2: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# ..$ a: num 1
# ..$ b: num 2
# $ a_3: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# ..$ a: num 1
# ..$ b: num 2
# $ b_1: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# ..$ a: num 1
# ..$ b: num 2
# $ b_2: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# ..$ a: num 1
# ..$ b: num 2
# $ c_1: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# ..$ a: num 1
# ..$ b: num 2
# $ c_3: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# ..$ a: num 1
# ..$ b: num 2
datspl <- split(dats, newxlsxfiles)
str(datspl)
# List of 3
# $ a:List of 3
# ..$ a_1: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# .. ..$ a: num 1
# .. ..$ b: num 2
# ..$ a_2: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# .. ..$ a: num 1
# .. ..$ b: num 2
# ..$ a_3: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# .. ..$ a: num 1
# .. ..$ b: num 2
# $ b:List of 2
# ..$ b_1: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# .. ..$ a: num 1
# .. ..$ b: num 2
# ..$ b_2: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# .. ..$ a: num 1
# .. ..$ b: num 2
# $ c:List of 2
# ..$ c_1: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# .. ..$ a: num 1
# .. ..$ b: num 2
# ..$ c_3: tibble [1 x 2] (S3: tbl_df/tbl/data.frame)
# .. ..$ a: num 1
# .. ..$ b: num 2
invisible( Map(writexl::write_xlsx, datspl, paste0(names(datspl), ".xlsx")) )
list.files(pattern = "xlsx$")
# [1] "a.xlsx" "b.xlsx" "c.xlsx"
readxl::excel_sheets("a.xlsx")
# [1] "a_1" "a_2" "a_3"
FYI: if you need to remove the leading a_
from the sheet names, just change the name of dats
before splitting it, perhaps
# ...
names(dats) <- sub("^[^_]*_", "", sheetnames)
datspl <- split(dats, newxlsxfiles)
# ...