Your increment of $MM applies to both values in the same command. Just use $i with a + 1 instead:

#!/usr/bin/env perl 
use strict;
use warnings;

for my $i (0 .. 9) {
    my $j = $i + 1;
    print "Input $i.mpg, output $j.mpg\n";
}
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Having the chdir and the glob inside the loop is pointless as they don't appear to be changing. And please put that long path into a variable instead of repeating it four times - typos will not be kind to you otherwise. Since you've changed directory already you can probably just use relative paths anyway.