Or use perlfunc:stat from within perl. It'll give you just about everything you wanted to know about a file and more. The times returned are in seconds-since-epoch format just like time() so a little arithmetic and a comparison is all you need.

# delete $file if it's not been modified for 3 hours
if ( (stat $file)[9] < time() - (3600 * 3) ) {
    unlink $file;
}
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