use strict;
use warnings;
use Text::CSV;
use MIME::Base64;

my $csv = Text::CSV->new( { binary => 1, eol => "\n" } );

while ( my $row = $csv->getline( \*DATA ) ) {
    for (@$row) {
        $_ = decode_base64($_) if m{
            ^
            (?: [A-Za-z0-9+/]{4} )*
            (?:
                [A-Za-z0-9+/]{2} [AEIMQUYcgkosw048] =
            |
                [A-Za-z0-9+/] [AQgw] ==
            )?
            $
        }x;
    }
    $csv->print( \*STDOUT, $row );
}
__DATA__
1,Just some data,c29tZSBzdHJpbmc=
2,VGhlIG5leHQgY29sdW1uIGlzIG5vdCBhY3R1YWxseSBCYXNlNjQgZW5jb2RlZA==,Oops