unlink $file if /.BGV/; ... what does this 'if' means? What condition is it?
The if after a statement is a statement modifier, it is the equivalent of if (/.BGV/) { unlink $file }. The condition is equivalent to $_ =~ /.BGV/, that is, match the special variable $_ against the regex /.BGV/. The special variable $_ gets set in several places, one very common one is the while (<$filehandle>) { ... } loop, see I/O Operators. That line of code in English: If the string stored in $_ contains the four-character sequence of any character (except newline) followed by the string 'BGV', then unlink (delete) the file whose name is given in the string $file.