I'm getting the same error in 5.26. What works is to use -B4 (i.e. no space).

Why?

$before = shift @ARGV assigns the first element of @ARGV to $before. The first element is -B, regardless of the Perl version. Also, changing the array you're currently iterating over doesn't seem to be a safe operation.

How to fix?

Iterate over the indices rather than elements if you want to peek into surrounding elements.

my $before;
my $arg_index = 0;
while ($arg_index < $#ARGV) {
    my $arg = $ARGV[$arg_index];
    if( $arg =~ /^-B(\d+)$/ ) {  # months Before
        $before = $1;
    } elsif( $arg =~ /^-B$/ ) {
        $before = $ARGV[++$arg_index];
        if( $before !~ /^\d+/ ) {
            die "Bad before argument ($before)\n";
        }
    }
} continue {
    ++$arg_index;
}
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Note that the code doesn't report "Bad before" if there's no space after -B but the value is not a number.

How to fix properly?

Use Getopt::Long.