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my $sql = "SELECT username FROM users WHERE username = ?";
my $user = $dbh->selectrow_array( $sql,{},$Username_POST );
if( $user ) { die "User '$user' exists; }
else { ... # user doesn't exist }

my $sql = "SELECT username FROM users WHERE username = ?";
my @row = $dbh->selectrow_array( $sql,{},$Username_POST );
if( @row ) { die "User exists"; }
else { ... # user doesn't exist }