Checking $! for errors is never a good idea unless you're sure the last syscall produced an error. $! may have any kind of value if the last syscall did not produce an error or is not documented to set $!.

As far as I can see the only way to check for this case is to see if the `` operator returned undef:

my $hostname = `hostname`;

unless (defined $hostname) {
    print "found error of: $!\n";
}
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update: as Fletch mentioned, `` does not set $! but $? so this code doesn't really work.

In addendum: be aware that not all functions that actually set $! are documented as doing so in the man pages. The only source that I know that seems to mention all of them is the Camel book.