$!
When referenced, $! retrieves the current value of the C errno integer variable. If $! is assigned a numerical value, that value is stored in errno . When referenced as a string, $! yields the system error string corresponding to errno .
Many system or library calls set errno if they fail, to indicate the cause of failure. They usually do not set errno to zero if they succeed. This means errno , hence $! , is meaningful only immediately after a failure:
if (open my $fh, "<", $filename)
