While previous posts prop better suggestions, another way is to declare @ARGV with local in a block so it reclaims the old values when it exits the block. As shown bellow, add lines #7, #8, and 19 to the old code:

my $arg;
my $argcount = 0;
my $debug = 0;
my $infile;

#line 7
{
#line 8 
        local @ARGV = @ARGV;

        while (@ARGV) {
            $arg = shift @ARGV;
            if ($arg =~ /-d/) {
                $debug = 1;
            } elsif ($argcount == 0) {
                $infile = $arg;
            }
        }
#line 19 
}
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