mtime as returned by (stat($filename))[9] is seconds since the epoch on your particular platform.
To see when the beginning of your machine's epoch was:

print scalar gmtime(0);
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Some modules useful for working with date/time:

• Date::Format
• Date::Calc
• Date::Manip

mtime is the number of seconds since 1970 (at least on UNIX-ish systems).
You can compare it to time() to see how old the file is.

my $mtime = (stat $filename)[9];
my $age = time() - $mtime;

die "HEY! $filename was created in the future!\n"
    if $age < 0;

print "$filename is $age seconds old\n";
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If you don't need the other stuff that comes from stat(), you may want to consider using the -M file test operator, which yields the age (since 'last modified') for the file, in (fractional) days.

if you want to know everything about the time, use the localtime function:

my $mtime = (stat $filename)[9];

my($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime($m
+time);
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