You've got two issues... first, getopts() requires you to specify a reference to a hash, not a hash directly (change %opts to \%opts in the getopts() call). Second, you're trying to assign $opts{f} to $file, but you're comparing with eq instead. Change eq to =. Here's the full code with fixes:

use Getopt::Std;

# declare the perl command line flags/options we want to allow
my %opts;
getopts('f:', \%opts);

# test for the existence of the options on the command line.
if ($opts{f}) {
        my $file = $opts{f};
        print "$file\n";
}
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