When faced with a complex regex, it is often better to generate it automatically:

my $test = 'vacation.msg';
my @alt;
foreach my $pos (1..length $test) {
   push @alt, substr $test, $pos; # prefix truncated
   push @alt, substr $test, 0, length($test) - $pos; # suffix truncate
+d
}
my $regex = join '|', $test, @alt;
$vacation_str = s/$regex//;
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You will have to alter the upper limit of the loop to determine how short of a string you want to stop at.

Update: fixed a typo.