Here's a complete solution to your problem. Assuming that the file that interests you is one of a group of files that are all in the form of mlb_boxscore$0000200.txt with the number portion 7 digits and zero-padded and the file you want always being the one with the hightest number, this will get you the name of the file you want:

opendir DIR, "/path/to/mlb/incomingdir";
my ($todays_file) = reverse sort readdir(DIR);
closedir DIR;
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If there are other files in the directory in question that we need to ignore, you'll need a little more:

opendir DIR, "/path/to/mlb/incomingdir";
my ($todays_file) = reverse 
                    sort 
                    grep {/mlb_boxscore\$\d{7}\.txt/} 
                    readdir(DIR);
closedir DIR;
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Now you can open $todays_file and process it as you like.

Sorry about the funny formatting of the crucial line to get it to fit here. If you like in your own code you can lay it all down in one line thusly: my ($todays_file) = reverse sort grep {/mlb_boxscore\$\d{7}\.txt/} readdir(DIR);