There's a space in the value of $date_time so your open() call thinks the filename is e:\scripts\OutageNodes\maintenanceMode_201586 and that you are passing 9:45:1.log as another argument. (Update: Problem is the colons, not the space as pointed out by marinersk, I was guessing at the exact cause of the error.)

Just use DateTime.

use DateTime;
my $dt = DateTime->now;
my $file = 'foo_' . $dt;
print $file;
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Prints:

foo_2015-08-06T17:05:17
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Note that this gives the datetime in ISO-standard format, which is good for later use elsewhere. Note also that now() returns a datetime in UTC timezone, so if you want your local time just use

$dt = DateTime->now( time_zone => 'local' );
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If you can't have colons in the file name, just specify a different format:

my $dt = DateTime->now(time_zone => 'local')->strftime("%Y-%m-%d-%k-%M
+-%S");
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Prints:

2015-08-06-10-42-18
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Finally note that $dt is a DateTime object but if you use it as a string it prints the datetime so you can be lazy.

Upate: showed DateTime use