The variable $. contains the line number of the last file read from, so:

for (<FILEHANDLE>);
print "FILEHANDLE contains $. lines";
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note: If you're using <> to iterate over @ARGV then the line numbers don't reset over the files. So on if you were reading five ten line files on line five of file three, $. will contain 25.

See perldoc perlvar for more info

Editor's Note:
$. is reset on an explicit close. Opening an open filehandle first does an implicit close on it, then opens it. This does NOT reset $. to zero. This is one of the many reasons to explicitly close filehandles when you are done with them. <> Uses the default filehandle and does not explicitly close it, which is why you get the behaviour that nuance mentioned. Of course, you can also set $. yourself as a point to start counting from (although this does NOT tell the read operator to start at the specified line, $. is just a counter) and you can scope $. with local $. as well.