PowerShell adds a few characters which you should not use in filenames. brackets and parentheses are the big ones. You can do it, but it's going to be a bit of a headache. I strongly suggest you not use them in filenames whenever possible.
Hmm. After looking at this for a minute, I think I agree that it is a bug. Get-Acl should have a Literalpath parameter. I would've expected this to work, but it does not:
PS > gi -LiteralPath `[`[`[.txt | Get-Acl
Get-Acl : The specified wildcard pattern is not valid: [[[.txt
At line:1 char:37
+ gi -LiteralPath `[`[`[.txt | Get-Acl <<<<
I searched Connect and did not see anything matching so I went and submitted a bug report. You should sign up at Connect yourself and vote for this bug so that it'll get more visibility.
sign up here bug report