You're reading file names from a directory and storing them in an array. If you want to operate on those file names, you need to include your file path with the file name in the operation.

#!/usr/bin/perl
  my $dir = "D:\\Log\\tmp";
  if(opendir LOGS, "$dir"){
   my @myfile = grep( /^*.tmp$/, readdir (LOGS));
   foreach $file(@myfile){
     my $result=-M "$dir\\$file";
     if( $result> 7){
     unlink ("$dir\\$file");
     }
    }
  }
  closedir LOGS;
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