That's why you salt it.

sub get_digest {
   my ($passwd) = @_;
   my $salt   = gen_8_rand_bytes());
   my $digest = Digest::SHA1::sha1_hex("$salt$passwd");
   return unpack('H16', $salt) . $digest;
}

sub cmp_digest {
   my ($passwd, $digest) = @_;
   my $salt = pack('H16', substr($digest, 0, 16, ''));
   return $digest eq Digest::SHA1::sha1_hex("$salt$passwd");
}
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There's probably an existing implementation on CPAN that follows established practices.