Always use strictures (use strict; use warnings;). The case error pointed out by jwkrahn would have been caught by use warnings;. Consider:

use strict;
use warnings;

my $msg = '9:23:17';

open (LOG, ">>", 'delme.txt');
print log "$msg\n";
close LOG;
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Prints:

Argument "9:23:17\n" isn't numeric in log at noname.pl line 7.
2.19722457733622
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which indicates you were double stung! log is a Perl built in function so definitely not a good name to chose! However, even if you had avoided the built in function:

use strict;
use warnings;

my $msg = '9:23:17';

open (LOG, ">>", 'delme.txt');
print Log "$msg\n";
close LOG;
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strictures would still save your bacon:

Name "main::Log" used only once: possible typo at noname.pl line 7.
print() on unopened filehandle Log at noname.pl line 7.
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And of course we also strongly recommend you use the three parameter version of open and check for success.