The shell (or whatever your command-line interpreter is) takes a solitary asterisk (*) command-line argument and turns it into a list of all of the filenames in the current directory. So, when you say rm *, you'll remove all of the files from the current directory. (Don't try this unless you like irritating your system administrator when you request the files to be restored.) Similarly, [a-m]*.c as a command-line argument turns into a list of all filenames in the current directory that begin with a letter in the first half of the alphabet and end in .c, and /etc/host* is a list of all filenames that begin with host in the directory /etc. (If this is new to you, you probably want to read some more about shell scripting somewhere else before proceeding.)
The expansion of arguments like * or /etc/host* into the list of matching filenames is called globbing. Perl supports globbing through a very simple mechanism: just put the globbing pattern between angle brackets or use the more mnemonically named glob function.
@a = </etc/host*>; @a = glob("/etc/host*");
In a list context, as demonstrated here, the glob returns a list of all names that match the pattern (as if the shell had expanded the glob arguments) or an empty list if none match. In a scalar context, the next name that matches is returned, or undef is returned if there are no more matches; this is very similar to reading from a filehandle. For example, to look at one name at a time:
while (defined($nextname ...