Because $, is the character used when you give a list of values to the print statement, but you've used '.'s.

Ie. You're giving the print statement a single (concatenated) value, therefore the item separator is never used.

This might achieve your goal:

 perl -lane '$,="\t"; print @F[0..4],  "\t\t\t",  @F[25..30]' file.txt

[download]

but "three blank columns" is a very nebulous concept.

Update: FTR, I might code that as:

print join "\t", @F[0..4], ('')x3, @F[25..30];

[download]
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The start of some sanity?

In reply to Re^5: Editing just one column in a file by BrowserUk
in thread Editing just one column in a file by ZWcarp

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