Shell error: unary operator expected"

I 've been learning Perl, and shell has forgotten about it. Today I wrote a small script to determine the operation when the second variable of the script is not equal to a certain string.

If [$? -EQ 0] & [$2! = "WP"]

Then

: # Omitted

Fi

If the second variable is not added [:! =: Unary operator expected "error, found during debugging

[! = "WP"]

If this problem occurs when shell variables are not defined, you can directly use double brackets.

[[! = "WP"]

Shell error: unary operator expected"