I want to do something like this: tr/\t/\s\s\s\s/ ## replace \t with 4 spaces

$ hexdump -C foo
00000000  61 09 62 62 62 09 63 63  63 63 63 63 09 64 0a     |a.bbb.ccc
+ccc.d.|
0000000f
$ perl -ple '$_ = join "    ", split /\t+/' foo > bar
$ hexdump -C bar
00000000  61 20 20 20 20 62 62 62  20 20 20 20 63 63 63 63  |a    bbb 
+   cccc|
00000010  63 63 20 20 20 20 64 0a                           |cc    d.|
00000018
$
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Edit: restricted now just to replacing tabs as this is presumably what was originally meant.