You can do it with the -M file test operator which returns result in floating point decimal days.

$dir = "c:/windows";
$age = -M $dir;

print "Dir $dir was last modified $age days ago\n";

# make a new dir just for fun
$dir = "c:/foo";
mkdir $dir;
$age = -M $dir;
print "Dir $dir was last modified $age days ago\n";
rmdir $dir; # delete this test dir
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With a dir the concept of modification is a little nebulous - it is actually the days since it was created.