If you want to do it perl5 way, you want Getopt::Std

use Getopt::Std;
my %opts;
getopt('i',\%opts);
die "usage $0 -i file" unless $opts{i} and -f $opts{i};
...
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See also The Dynamic Duo --or-- Holy Getopt::Long, Pod::UsageMan!, and Getopt::Long::Descriptive